Beal attended Chaminade College Preparatory School in St. Louis, Missouri. He competed in the 2010 FIBA Under-17 World Championship for the USA, which won the championship with Beal averaging 18 points per game. During his senior year of high school, Beal averaged 32.5 points per game, 5.7 rebounds per game and 2.8 assists per game. At the end of Beal's senior season, he was named the 2011 Mr. Show-Me Basketball which recognized him as the top high school basketball player in the state of Missouri. He was also named the 2011 Gatorade National Player of the Year.
On November 30, 2009, Beal committed to the University of Florida. Beal accepted an athletic scholarship to attend the university, where he played for coach Billy Donovan's Florida Gators men's basketball team during the 2011–12 season.
In his first game at Florida, Beal started and recorded 14 points. Beal was named SEC Freshman of the Week on November 28, 2011. During the week of 11/21–11/28, Beal averaged 8.5 points, seven rebounds, two assists and 1.5 steals in two wins over Wright State and Jacksonville. Beal would go on to win five additional SEC Freshman of the Week honors and be named to the SEC All-Freshman Team and was a first-team All-SEC selection. Beal finished the season averaging 14.8 points per game. He also helped his team advance to the Elite Eight during the NCAA tournament before being ousted by Louisville.
On April 13, 2012, Beal declared for the NBA draft, foregoing his final three years of college eligibility. On June 28, 2012, Beal was drafted with the third overall pick in the 2012 NBA draft by the Washington Wizards.
Beal was named the Eastern Conference Rookie of the Month in December 2012 and January 2013. On January 4, 2013, in a game against the Brooklyn Nets, Beal scored a then career high 24 points and hit a clutch three-pointer to send the game to double overtime; the Wizards went on to lose the game. On January 17, Beal set a career best with six three-pointers made in a loss to the Sacramento Kings. He went on to be selected to appear in the Rising Stars competition at the 2013 All-Star weekend.
On April 3, 2013, it was announced that Beal would miss the remainder of the 2012–13 season with a right leg injury. At the end of the season, after playing 56 games, he was named to the NBA All-Rookie first team, and finished third in voting for the NBA Rookie of the Year award.
On November 10, 2013, Beal surpassed his career-high by scoring 34 points in an overtime loss to the Oklahoma City Thunder. He later set a new benchmark for his personal best by scoring a career-high 37 points in a loss to the Memphis Grizzlies. Beal finished as runner-up in the Three-Point Shootout at the 2014 NBA All-Star Weekend in New Orleans, losing to Marco Belinelli. At the time, Beal was the youngest competitor in the event's history. On April 29, 2014, Beal and the Wizards defeated the Bulls in the first round of the Eastern Conference Playoffs, moving the team on to the second round, a feat the franchise hadn't reached since 2005.
On October 11, 2014, Beal underwent an MRI that revealed a non-displaced fracture of the scaphoid bone in his left wrist, subsequently requiring surgery. He was later ruled out for six to eight weeks. After missing the first nine games of the season with the injury, he made his season debut on November 19 against the Dallas Mavericks. In just under 26 minutes off the bench, he recorded a team-high 21 points, as well as 3 rebounds, 3 assists and 1 steal, in the 102-105 loss. He went on to make a game-winning buzzer beater on December 10 against the Orlando Magic. With 0.8 second left on the clock, coach Randy Wittman drew up a guard around screen for Beal off the pass from Andre Miller that ended Orlando's attempts to send the game into overtime. He finished the game with 9 points, 3 rebounds, 3 assists and 1 block in the 91-89 win.
On February 5, Beal injured his right big toe and was later sidelined indefinitely after subsequent tests revealed a mild stress reaction in his right fibula. He missed eight games with the injury, as he returned to action on February 28 against Detroit, scoring eight points in 32 minutes as the Wizards snapped a six-game losing streak with a 99-95 win.
In Game 1 of the Wizards' semi-final match-up against the Atlanta Hawks on May 3, Beal scored a playoff career-high 28 points despite spraining his ankle early in the fourth quarter, helping his team defeat the Hawks 104-98. In Game 4 of the series on May 11, Beal scored a new playoff career-high with 34 points in a loss to the Hawks which tied the series up at 2-2. The Wizards went on to lose the series to the Hawks in six games.
On November 4, 2015, Beal scored a game-high 25 points and made a three-pointer with 0.3 seconds remaining to lift the Wizards to a 102–99 win over the San Antonio Spurs. Beal had an injury-plagued season in 2015–16, as he missed three games in mid-November with a shoulder injury, and 16 games between December 11 and January 11 with a lower right leg injury. He had another stint on the sidelines in early March, missing three games with a sprained pelvis. He played in a career-low 55 games in 2015–16, and made a career-low 35 starts, but did record a career-high 17.4 points per game.
On July 26, 2016, Beal re-signed with the Wizards. On November 19, 2016, he scored 34 points in a 114–111 loss to the Miami Heat. Two days later, he had 30 or more in consecutive games for the first time in his five-year career, recording a career-high 42 points in a 106–101 win over the Phoenix Suns. On November 27, he was fined $15,000 for grabbing at the throat of Evan Fournier during the Wizards' game against the Orlando Magic two days earlier. On November 28, he scored 31 points and hit a career-high seven three-pointers in a 101–95 overtime win over the Sacramento Kings. On December 14, he recorded 20 points and a career-high nine assists in a 109–106 win over the Charlotte Hornets. With two made three-pointers on December 16 against the Detroit Pistons, Beal reached 501 career three-pointers and became the third Wizards player to make 500, joining Gilbert Arenas and Antawn Jamison. On December 18, he had a 41-point effort in a 117–110 win over the Los Angeles Clippers. On February 6, 2017, he had another 41-point effort in a 140–135 overtime loss to the Cleveland Cavaliers. On February 24, 2017, he scored 40 points in a 120–112 loss to the Philadelphia 76ers, recording his fourth 40-point game of the season. On March 29, 2017, he scored 27 points in a 133–124 loss to the Los Angeles Clippers. During the game, Beal surpassed Gilbert Arenas for the franchise single-season three-point record. He made five against the Clippers to finish with 209 by the end of the game—Arenas had 205 in 2004–05 and 2006–07. Beal joined Kevin Durant and Vince Carter as the only three players in NBA history, 23 years old or younger, to average at least 23 points and shoot above 40 percent from three-point range.
On May 12, 2017, Beal helped the Wizards avoid elimination and force a Game 7 in their second-round playoff series against the Boston Celtics with a game-high 33 points in a 92–91 Game 6 win. In Game 7 three days later, Beal led the Wizards—a team playing in its first Game 7 since 1979—with 38 points, including 24 in the second half, but he couldn't lead them to victory as they fell 115–105 to bow out of the playoffs with a 4–3 defeat.
Beal is the son of Bobby and Besta Beal, and has four brothers: Brandon, Bruce, Byron and Bryon. All four played or are currently playing college football: Brandon played tight end at Northern Illinois; Bruce played offensive lineman at Alabama State; and Bryon and Byron play offensive linemen and defensive line at Lindenwood University. Rapper Nelly, who is a family friend, used to walk Beal to school.
During his time at Florida, Beal dated Kytra Hunter of the Florida Gators gymnastics team.