Sims was a 2010 graduate of MacArthur High School in Irving, Texas. She was rated the number one point guard in the class of 2010, earned the WBCA/State Farm National High School Player of the Year award and had her jersey retired.
Sims attended Baylor University for four seasons. As a freshman, Sims was Named National Freshman of the Year, was named Big 12 Freshman of the Year and also to the All-Big 12 first team. In her sophomore season, Sims earned team Co-MVP honors with teammate Brittney Griner. Scored in double-digits in 30 of 40 games and won a NCAA Championship after Baylor defeated Notre Dame. In her junior year, Sims was earned Co-MVP honors for the second straight year with Griner. In Big 12 statistics, she was ranked number 1 in assist/turnover ratio (2.7), number 3 in steals (2.8), number 4 in assists (5.8), and number 5 in free throw percentage (.832). In her senior year, Sims would statistically have the best season of her college career. She scored a career-high 48 points along with 10 rebounds in a 78-62 victory against West Virginia. She finished the season averaging 28.5 points per game.
Sims was drafted 2nd overall by the Tulsa Shock in the 2014 WNBA draft. In her rookie season, Sims immediately entered the starting line-up for the Shock at the point guard position. Sims would instantly become a scoring sensation in the league; she scored a career-high 39 points in a loss to the San Antonio Stars. She finished her rookie season, averaging a career-high 16.7 points per game along with a career-high 4.2 assists per game in 34 games with 31 starts and was named to the WNBA All-Rookie Team.
In 2015, Sims would have an injury-riddled season, she missed 10 games with a knee injury. She finished the season, with 23 games played along with 19 starts and averaged 16.0 points per game and 3.8 assists per game. In 2016, the Tulsa Shock relocated to Dallas, Texas (which is approximately 15 miles away from Sims's hometown of Irving, Texas) and were renamed the Dallas Wings.
In the 2016 season, Sims would be healthy; she played in 34 games with 30 starts and averaged 14.0 points per game along with 3.9 assists per game at shooting guard.
In 2017, Sims was traded to the Los Angeles Sparks along with the number 11 overall pick in the 2017 WNBA Draft in exchange for two first round picks in the 2017 WNBA Draft. She would share the starting shooting guard role with Essence Carson during the season. On August 24, 2017, Sims scored a season-high 28 points along with 8 assists and 6 rebounds in the starting lineup in a 82-67 victory over the Phoenix Mercury. The Sparks would finish with a 26-8 record as the number 2 seed in the league with a double-bye to the semi-finals. Heading into the playoffs, Sims would be starting shooting guard for the Sparks. In her first career-playoff game, Sims scored 12 points along with 5 assists in a 79-66 Game 1 win over the Phoenix Mercury. The Sparks would go on to advance to the Finals after defeating the Mercury in a 3-game sweep. Sims scored a playoff career-high 22 points in Game 3 with the Sparks winning 89-87 to clinch their second consecutive berth to the Finals.
In the 2014-15 WNBA off-season, Sims played for Bucheon KEB-Hana Bank in South Korea. In the 2015-16 WNBA off-season, Sims played for Abdullah Gül Üniversitesi B.K. in Turkey. As of August 2016, Sims signed with Botaş SK of the Turkish League for the 2016-17 WNBA off-season.
Sims played on the team representing the USA at the 2011 Summer Universiade held in Shenzhen, China. The team, coached by Bill Fennelly, won all six games to earn the gold medal. Sims averaged 6.2 points per game.
Sims was selected to be a member of the team representing the USA at the 2013 Summer Universiade held in Kazan, Russia. The team, coached by Sherri Coale, won the opening four games easily, scoring in triple digits in each game, and winning by 30 or more points in each case. After winning the quarterfinal game against Sweden, they faced Australia in the semifinal. The USA team opened up as much as a 17 point in the fourth quarter of the game but the Australian team fought back and took a one-point lead in the final minute. Crystal Bradford scored a basket with 134 seconds left ant he game to secure a 79–78 victory. The gold medal opponent was Russia, but the USA team never trailed, and won 90–71 to win the gold medal and the World University games Championship. Sims was the third leading scorer for the team, averaging 12.7 points per game. She led the team in assists with 32, and steals with 12. She was named co-MVP of the tournament, along with Russia's Tatiana Grigoryeva.2010—WBCA High School Coaches' All-America Team
2014—ESPNW First Team All-American
2014—USBWA All-American team
2014—Nancy Lieberman Award
2014—Frances Pomeroy Naismith Award
2014—Dawn Staley Award