Stewart was born in Syracuse, New York, to Brian and Heather Stewart and has a younger brother named Connor. Breanna began playing basketball at an early age; in fifth grade, she decided to improve her game. Now six feet four inches tall, she had always been tall for her age, which often meant coaches wanted her inside as a rebounder. However, her father thought it would help if she had ball handling skills and a perimeter shooting ability. Stewart started a routine of dribbling around her block, wearing headphones. She completed enough loops to cover a mile. She continued the routine almost every day, improving her ball handling to the point she routinely dribbled behind her back or between her legs. Even after heading off to college, she still did the routine at home.
Stewart attended Cicero-North Syracuse High School (C-NS) in Cicero, New York, where she played for head coach Eric Smith. She was nicknamed "Bean" by her teammates, and "6-10" because of her wingspan. Stewart first played for the high school team while still in eighth grade. She played as a starter in most games, and averaged nine points, almost nine rebounds and seven blocks per game. In her freshman year, she almost doubled her point production, scoring 17 points per game. That year, her team had a 21–3 record, and made it to the regional final game.
In her sophomore year, she was a starter in every game, and upped her scoring average to 22 points per game. In that year, her team's record was 18–4. As a junior, she helped lead her team to the state AA public school title, with a 22–3 record for the year. Stewart averaged 24 points and 15 rebounds for the season. During her junior year, she announced that she would be attending the University of Connecticut. The day after the announcement, she dunked the ball in a game against Baldwinsville, her first career dunk. Stewart achieved a milestone on January 31, 2012, when she scored her 2,000th point, as part of a 31–0 run against Auburn. She amassed 2,367 points, 1,389 rebounds, 337 assists, 325 steals and 634 blocks in 119 games over five varsity seasons at Cicero-North Syracuse. Stewart led the Northstars to an 84-13 record (.866) over the last four seasons, including winning the state Class AA Federation championship.
Stewart was selected as a member of the 2012 McDonald's All-American team, which represented the 24 best female high school basketball players. The selected players were grouped into two squads that competed in the annual McDonald's All-American Game, held that year in Chicago. Stewart was selected to the 2012 Women's Basketball Coaches Association (WBCA) High School Coaches' All-America Team. The top 20 high school players in the country were named as WBCA All-Americans and were eligible to play in the all-star game. She participated in the 2012 WBCA High School All-America Game, scoring 10 points. Stewart was named the 2012 Naismith High School Girls' Player of the Year, the honor awarded by the Atlanta Tipoff Club to the best female high school basketball player in the country. In March 2012, in a surprise presentation by Tamika Catchings, Stewart received the Gatorade National Girls Basketball Player of the Year award. Stewart was one of six finalists for the Gatorade High School Athlete of the Year.
In addition to regularly scheduled post-season tournaments, the success of her high school team led to invitations to prominent national tournaments. In 2010, the C-NS team traveled to Philadelphia, New Jersey and Disney World; in 2011, the team went to the Nike Tournament of Champions in Phoenix, Arizona, considered the "premier showcase of all high school girls tournaments"; and in 2012, the Northstars played in a Basketball Hall of Fame Tournament in Springfield, Massachusetts.
The Tournament of Champions is an annual event, since 1997, showcasing the best high school girls basketball teams. The 2011 event, held in Phoenix, Arizona, included 96 of the best basketball programs in the country. The C-NS team was assigned to the Smith Division, where they faced Bolingbrook High School, considered to be the number 1 team by USA Today. Despite being viewed as a 30-point underdog, Stewart helped her team to the first round upset, scoring 15 points in a 43–40 win. In the quarter-final game, Stewart scored 29 points and had 19 rebounds to help the team beat the number 22 ranked team in the country, Dr. Phillips High School from Orlando, Florida. Although double-teamed, she scored ten points in a 12–0 run that gave C-NS a commanding 23-point lead early in the fourth quarter. That win secured a place in the semi-finals of the division. In the semi-final game, C-NS faced St. Mary's, the number 2 ranked team in the country. Stewart had 33 points and 16 rebounds, but it was not enough to overcome the scoring of the eventual champion of the tournament. Although the team lost that game, Stewart's overall performance earned her the most outstanding player of the tournament honors. Stewart was viewed as an offensive threat in the low post, high post and perimeter, and was effective as a defender because of her long wingspan.
Stewart's Northstars team were invited to play in the annual Hoophall Classic; the event was held in Springfield, Massachusetts, and typically featured some of the best high school teams in the country. Their opponents were the local West Springfield team, not nationally ranked, but 8–0 at the time of the meeting. The result was not close, as C-NS beat West Springfield 60–20, with Stewart alone scoring more points than the opposition. Stewart ended the game with 22 points, 18 rebounds and seven blocks, despite leaving the game in the third period and not returning. Her performance earned her the Most Outstanding Player award for the event.
Stewart was selected to join the USA Basketball U16 team, at age 14, the youngest member of the team; all other team members were 15 or 16. Initially, her parents turned down the invitation to join the team, worried about the amount of time she would be missing school. However, Mike Flynn, director of a prominent Amateur Athletic Union team, persuaded her parents that the invitation was an honor, so they relented. At the time, she stood six feet, three inches, the same height as Kiah Stokes and Elizabeth Williams. Despite being the youngest, she earned the starting role for all five games, scoring just under ten points per games, tying Elizabeth Williams for the team lead in blocks. Stewart helped the team win the gold medal in the First FIBA Americas U16 Championship for Women, held in Mexico City, Mexico. The win secured an automatic bid to the 2010 FIBA U17 World Championship, held in Rodez and Toulouse, France.
Stewart continued with the USA team as it became the U17 team. The USA team won all eight games and the gold medal in the 2010 FIBA U17 World Championship for Women. Stewart earned a starting role in all eight games. In the first game, against the host team from France, she led all scorers with 13 points. In the final preliminary game against Japan, she led all scorers with 30 points. In the tournament, she averaged 12.8 points per game, second only to Elizabeth Williams at 13.5 points per game. Stewart was the team leader with 18 blocks.
In 2011, Stewart was selected to be on the USA Basketball U19 team. The U18 team won the 2010 Americas Championship, thus earning an automatic bid to the U19 World Championship, held in Puerto Montt, Chile. The USA won their first five games, but then came up short, losing to Canada 64–52. They were still qualified for the medal round, and played against France in the quarter-final; the USA was down by 13 points early in the game, but took a lead with just over a minute to go in the game and won 70–64. The USA took an early lead in the semi-final against Brazil, and qualified for the gold medal game. The final was against Spain, which the USA won 69–46. Although she was one of the youngest players on the team, Stewart averaged 11.2 points per game to post the highest scoring average of the USA players. She was one of the five players named to the all tournament team; Ariel Massengale was the other USA player to earn all tournament honors.
She competed for the United States at the 2011 Pan American Games. The USA Pan American team members were usually chosen from the college ranks, although many of the other countries use their national teams, which include professional players. Stewart was the only high school player chosen for the 2011 Pan American team, and only the second high school player in Pan American team history for the USA teams. The only other high school player on a Pan American team was Nancy Lieberman, who played on the 1975 team, before Stewart was born.
The 2011 team finished seventh, the first time in history they did not earn a medal, but Stewart, almost three years younger than the next youngest player, was still a major contributor, scoring 15.4 points per game to lead the team in scoring. She also led the team in blocks and rebounds.
Stewart was named the 2011 USA Basketball Female Athlete of the Year. USA Basketball cited her performance on the U19 team and the Pan American team. She was the second youngest on the U19 team, yet led the team in points, rebounds and blocks, helping to lead her team to a gold medal at the international competition, and earning a position on the all-tournament team. She occupies several spots in the U19 record books. She was named to the Pan American team, only the second high school player from the USA to receive such a bid. Despite playing with college age players, she earned a starting position for all games, and again led the team in points, rebounds and blocks. The previous year’s winner of the award was Diana Taurasi.
Stewart was named to the USA Basketball U-18 team. She joined future UConn teammates Moriah Jefferson and Morgan Tuck on the 12-player squad that competed in the 2012 FIBA Americas U-18 National Championship, held in August 2012, in Puerto Rico. Stewart was the youngest, and only high school player, on the 2011 USA Women's Pan American Games team, but was the player with the most international experience on the team. The team started by winning their first four games with margins of victory of 40 points or more. This set up the championship game against Brazil. The team from Brazil started strong, and held a double-digit lead early in the game. After scoring only seven points in the first quarter, the team scored 21 or more in the next three quarters and reclaimed the lead. The USA won 71–47 to claim the gold medal.
Stewart was awarded the MVP trophy as the best player in the FIBA Americas U18 competition.
Stewart, along with teammates Moriah Jefferson and Morgan Tuck, were three of the twelve players selected to be on the team representing the USA at the U19 World Championship for Women held in Klaipėda and Panevėžys, Lithuania, in July 2013.
Stewart was named the USA Basketball Female Athlete of the Year for the second time. Only five other players have won this award more than once, Teresa Edwards, Lisa Leslie, Cheryl Miller, Dawn Staley and Diana Taurasi, and none have accomplished this at so young an age. Stewart helped the USA to a 9–0 record in the 2013 FIBA U19 World Championship, leading to a gold medal for the team and MVP honors for Stewart.
Stewart was one of 33 finalists for the U.S. Women's FIBA World Championship for Women Roster. She was the only collegiate player out of 32 professional women's basketball players to be selected by the USA Basketball Women’s National Team Player Selection Committee to compete for the final roster to represent the USA at the FIBA World Championship for Women in Turkey during September and October 2014. Stewart made the final roster of 12 players, announced on September 23, 2014. Stewart won the gold medal with the team in 2014.
Stewart was a member of the USA Women's Pan American Team which participated in basketball at the 2015 Pan American Games held in Toronto, Ontario, Canada July 10 to 26, 2015. The USA won games against Brazil, the Dominican Republic, Puerto Rico, and Cuba, before losing the gold-medal game to the host team Canada.
Stewart was selected to play on the USA Women's National Team for the 2016 Summer Olympics despite being a rookie in the WNBA. The roster for the team had been announced before Stewart played her first WNBA game. Stewart averaged 13.7 points per game with 13.1 minutes per game of playing time, helping Team USA advance all the way to the gold medal match, defeating Spain 101-72 for the gold medal.
Stewart was recruited by many schools, but after a campus visit to UConn in 2011, she told the coaching staff, "I would like to commit if you'd take me". Committing requires a formal letter of intent. Some players sign at a media event organized to cover the ceremony; Stewart signed her letter on the hood of her car, then gave it to her father at his office to fax to the school.
Stewart started her freshman year in strong fashion, scoring at least 20 points in three of her first four games. She scored a total of 169 points in her first ten games, which is more than any other Connecticut Husky freshman. However, her output slowed, and her scoring average dropped below ten points for her last eighteen regular season games. In March, she started early morning session with Chris Daily, associate head coach, to concentrate on shooting and post moves. She came back strong in the Big East tournament, with a total of 51 points, matching the number scored by Diana Taurasi in her tournament debut. Her strong performance continued in the NCAA tournament. She did not play in the first round, due to a calf injury, but scored 105 points in the last five games, earning her the award of the Most Outstanding Player of the Final Four, the first freshman to win the award since 1987.
Stewart continued her stellar play in her sophomore year; her coaches noticed that she was demanding the ball more often, something she occasionally did as a freshman, but usually only if she thought she was playing well. By the end of her sophomore year, she was named the American Athletic Conference Player of the Year in the league's first season after the split of the original Big East Conference, an honor she would go on to win the following two seasons. Additionally, she was named AP Player of the Year, only the third time in history a sophomore had claimed the honor. The other two sophomore recipients were UConn's Maya Moore and Oklahoma's Courtney Paris. Stewart started and played in all 40 games of 2013–2014 season. She led the team at 19.4 points and 2.8 blocks per game, was second on the team with 8.1 rebounds per contest, and fourth on the squad at 49.7 percent shooting and her 291 field goals made was the third-highest single-season total in UConn annals. Her 324 boards marked the 12th-highest single-season total in school history, and her 110 rejections was the third-highest mark.
Stewart earned American Athletic Conference Player of the Year honors for the second straight year in 2014-15, marking the fifth time a Husky was selected as the conference player of the year multiple times. She elevated nearly every aspect of her game during UConn's nine-game run through the postseason, averaging 18.1 points on 53.3 percent shooting to go along with 10.7 rebounds, and made 42.1 percent (8-19) of her attempts from beyond the 3-point arc during postseason action. She entered her senior year in 11th place on UConn's all-time scoring list with 1,960 career points, only 30 points behind Renee Montgomery's 10th-place total. She entered 2015-16 in fifth place on UConn's all-time blocked shots list with 288 career rejections and finished the season with 856 career rebounds, which is the 10th-highest total in UConn history.
In her final year at UConn, Stewart had an amazing season; Stewart posted career highs in rebounds (8.7 rpg), assists (4.0 apg) and blocks (126) while she shot 57.9 percent from the floor, sweeping all possible individual honors: she won her 3rd straight Naismith College Player of the Year award, Wade Trophy, Associated Press Women's College Basketball Player of the Year award, USBWA Women's National Player of the Year award; she was also voted American Athletic Player of the Year (3rd time) and a third straight unanimous First-Team All-American in WBCA, USBWA and AP polls. She was the first-ever unanimous pick for AP Player of the Year and the first-ever three-time AP Player of the Year. In leading UConn to another national championship, she was part of the first four-time national championship class in NCAA history and also became the first player to be named the Final Four Most Outstanding Player four times. In her amazing college career Stewart won 151 games and only lost 5, which included four straight National Championships. She is the first NCAA basketball player ever to tally 300 assists and block 300 shots. She was one of only six Huskies, along with Tina Charles, Rebecca Lobo, Maya Moore, Stefanie Dolson, and Jamelle Elliott, with at least 1,000 points and 1,000 rebounds. Stewart finished her career second on UConn's all-time scoring list at 2,676 career points, fourth with 1,179 career rebounds, first in blocks (414) and fifth in field goal percentage (53.0 percent).
Stewart was drafted first overall in the 2016 WNBA Draft by the Seattle Storm. She would play alongside superstar veteran point guard Sue Bird. Stewart immediately made an impact in the league as she scored 23 points in her debut game against the Los Angeles Sparks. As the season progressed, Stewart continued to dominate offensively as she scored a career-high 38 points in a win against the Atlanta Dream, which is the second most points scored in a game in franchise history, behind Lauren Jackson's 47 points. Stewart averaged 18.3 points per game, 9.3 rebounds per game and 1.8 blocks per game by the end of the 2016 season. She swept the Rookie of the Month awards for the entire season leading up to her winning the WNBA Rookie of the Year Award by a landslide. Stewart also broke the WNBA record for most defensive rebounds in a season with 277 defensive rebounds (surpassing Lisa Leslie's record in 2004). Despite the record, Stewart didn't win the rebounding title, sharing the top spot with Tina Charles for most rebounds, who led the league in rebounds per game average. Her season performance helped lead the Storm back to the playoffs for the first time in 3 years with the number 7 seed in the league, but lost in the first round elimination game to the Atlanta Dream. Stewart won the 2016 ESPY Award for Best Female Athlete. The other nominees were Elena Delle Donne, Katie Ledecky, and Simone Biles. She also won Best Women's College Basketball Player at the 2016 ESPYs.
In 2017, Stewart continued to flourish after an impressive rookie season. She was voted into the 2017 WNBA All-Star Game, making it her first career all-star game appearance. On August 5, 2017, Stewart scored a season-high 32 points in a 87-80 overtime loss to the San Antonio Stars. Stewart would finish the season with a career-high of 19.9 points per game as the Storm finished with the number 8 seed in the league, but were once again a first-round exit as they were eliminated by the Phoenix Mercury.
During her rookie season, Stewart signed with Shanghai Baoshan Dahua of the WCBA for the 2016–17 off-season.2010—ESPN Rise All-America second team
2010—Parade All-America fourth team
2011—Gatorade New York Girls Basketball Player of the Year
2011—All-FIBA U19 World Championship Team
2011—USA Basketball Female Athlete of the Year
2012—WBCA High School Coaches' All-America Team
2013—Most Valuable Player award at the 2012 Paradise Jam Tournament
2013—Most Outstanding Player of the Final Four, the first freshman to win the award since 1987.
2013—NCAA Final Four All-Tournament team.
2013—ESPNW 2013–14 preseason player of the year
2013—USA Basketball female athlete of the year
2014—American Athletic Conference Player of the Year
2014—ESPNW player of the year
2014—ESPNW First team All-American
2014—USBWA All-American team
2014—AP All-American First Team
2014—AP Player of the Year
2014—Women's Naismith College Player of the Year
2014—Ann Meyers Drysdale Award
2014—Winner of the Honda Sports Award for basketball
2014—Most Outstanding Player of the Final Four
2015—Most Outstanding Player Albany Regional
2015—AP Player of the Year
2015—Wade Trophy winner
2015—USBWA All-American team
2015—Ann Meyers Drysdale Award
2015—Women's Naismith College Player of the Year
2015—Most Outstanding Player of the Final Four (making her the first woman to be most outstanding player of the Final Four three times)
2015—Winner of the Honda Sports Award for basketball
2015—AP All-American First Team
2016—AP All-American First Team
2016—Most Outstanding Player Bridgeport Regional
2016—AP Player Of The Year (making her the first female college basketball player to win that award three times)
2016—Women's Naismith College Player of the Year
2016—USBWA All-American team
2016—Ann Meyers Drysdale Award
2016—Most Outstanding Player of the Final Four (making her the first person to be most outstanding player of the Final Four four times)
2016—John R. Wooden Award
2016—James E. Sullivan Award
2016—Winner of the Honda Sports Award for basketball
2016—Winner of the Honda-Broderick Cup and named Collegiate Woman Athlete of the Year
2016—ESPY for best female athlete
2016—WNBA Rookie of the Year
Stewart attended a rally at Los Angeles International Airport against Trump's immigration executive order. She considered her first protest "inspiring".