The third of five children, Allen was born at Castle Air Force Base near Merced, California, the son of Walter Sr. and Flora Allen. A military child, he spent time growing up in Saxmundham in England, Altus in Oklahoma, Edwards Air Force Base in California, and Germany. After years of traveling and constant moving, he moved to Dalzell, South Carolina where he would attend high school for the next four years. When he first arrived, he was always the odd man out that kids often picked on for his formative language that he used due to his elementary years in Britain. While he never completely fit in with the other kids, his natural athletic gift and obsession with hard work allowed him to excel in every sport he played. After a growth spurt gave him an advantage in basketball, he decided to dedicate all his time to become the best basketball player he could be. Fueled by becoming the best basketball player on the military base where he lived, Allen practiced at length everyday as long as it didn't interfere with his studies. By the age of fifteen, he was playing for Hillcrest High School's varsity team and would eventually lead them to their first state championship game. In that game, Allen would show his NBA potential by posting an impressive 25 points to go along with 12 rebounds in a blowout victory for Hillcrest Wildcats. With a lot of college attention, especially from the University of Kentucky, Allen would ultimately decide to go to the University of Connecticut.
Allen attended the University of Connecticut from 1993 to 1996 after being recruited by assistant coach Karl Hobbs. While at UConn, he was named USA Basketball's Male Athlete of the Year in 1995. In 1995–96, his final college season, Allen was a first-team All-American and won the Big East Player of the Year award. Allen finished his UConn career third on the Huskies' career scoring list with 1,922 points and set a single-season school record by connecting on 115 three-pointers in 1995–96.
In 2001, Allen was named honorary captain of the 25-member UConn All-Century Basketball Team. On February 5, 2007, his name and number were honored at Connecticut's Gampel Pavilion during the "Huskies of Honor" ceremony at halftime of the men's basketball game against the Syracuse Orange.
Allen was drafted by the Minnesota Timberwolves with the fifth pick of the 1996 NBA draft. Immediately after his selection, Allen and Andrew Lang were traded to the Milwaukee Bucks for the rights to fourth pick Stephon Marbury. Allen made his NBA debut on November 1, 1996 where he started and played 28 minutes and scored 13 points in a win against fellow rookie Allen Iverson and the Philadelphia 76ers. On January 12, 1997 Allen put in one of his strongest efforts of the season in a win against the Golden State Warriors contributing 22 points, 6 assists, 3 steals and a new career high of 9 rebounds. Continuing his strong rookie season, on March 25, 1997 Allen scored a new career high of 32 points in a loss to the Phoenix Suns. Allen was named to the NBA All-Rookie Second Team in 1996. His most successful season with the Bucks occurred during the 2000–01 season: he won the 3-point shootout during All-Star Weekend; was selected to the All-NBA Third Team; and led the Bucks, as part of Milwaukee's "Big Three", alongside Sam Cassell and Glenn Robinson, to the Eastern Conference Finals, where they lost in seven games to the Allen Iverson-led Philadelphia 76ers.
Allen remained with the Bucks midway through the 2002–03 season, when he was dealt to the Sonics, along with Ronald Murray, former UConn teammate Kevin Ollie, and a conditional first round draft pick, in exchange for Gary Payton and Desmond Mason. After an injury-riddled 2003–04 season, he was named to the All-NBA Second Team and, alongside teammate Rashard Lewis, led the Sonics to the Conference Semifinals in 2005.
During the 2004 preseason, Allen had a brief war of words with Los Angeles Lakers guard Kobe Bryant, whom Allen accused of alienating teammates trying to prove that he did not need Shaquille O'Neal to win games and championships. Allen told the press that if the Lakers remained a mediocre squad, "in about a year or two he will be calling out to (Lakers owner) Jerry Buss that we need some help in here, or trade me." When asked about Allen's comments, Bryant responded, "Don't even put me and that dude in the same breath."
After the 2004–05 season, Allen signed a 5-year, $80 million contract extension. In the 2006–07 regular season, he averaged a career-high 26.4 points per game while adding 4.5 rebounds and 4.1 assists per game.
During his Seattle SuperSonics tenure, Allen achieved many individual accomplishments. On March 12, 2006, Allen became the 97th player in NBA history to score 15,000 points. On April 7, 2006, Allen moved into second place on the NBA's list of all-time 3-point field goals made, trailing only Reggie Miller. On April 19, 2006, Allen broke Dennis Scott's ten-year-old NBA record for 3-point field goals made in a season in a game against the Denver Nuggets. The record has since been broken by Stephen Curry.
On January 12, 2007, Allen scored a career-high 54 points against the Utah Jazz in a 122–114 overtime win, the second most in Sonics history. Shortly after, he had ankle surgery on both ankles and missed the remainder of the 2006–07 season.
On June 28, 2007, the Sonics traded Allen and Glen Davis, the 35th overall pick in the 2007 NBA draft, to the Celtics in exchange for Delonte West, Wally Szczerbiak, and the fifth overall pick, Jeff Green. Shortly after acquiring Allen, the Celtics acquired Minnesota Timberwolves forward Kevin Garnett to play alongside Allen and Paul Pierce. Upon joining the Celtics, Allen changed his jersey to the number 20, as Paul Pierce already had the number 34 in Boston.
On November 4, 2007, Allen passed 17,000 points for his career with his first of two 3-pointers in overtime in a 98–95 victory against the Toronto Raptors, in which he sank the game winning 3-pointer with three seconds remaining in overtime.
On February 13, 2008, Allen was named as the replacement for injured East All-Star Caron Butler of the Washington Wizards, who was out with a left hip flexor strain, for the 2008 NBA All-Star Game in New Orleans. While LeBron James was given the All-Star MVP Award, many analysts, including the TNT commentators of the game, felt it should have gone to Allen, who scored 14 points in a stretch of 2 minutes and 30 seconds in the fourth quarter to seal the win for the East team.
On March 28, 2008, Allen was honored as the 3rd best of the 20 greatest players in franchise history during Milwaukee's 40th Anniversary Team Celebration, but could not attend the festivities because of the Celtics' game against the New Orleans Hornets. The Boston Celtics finished 66–16, and were the #1 seed in the 2008 NBA Playoffs. The Celtics struggled in the first two rounds of the playoffs, losing every single away game, but maintaining an undefeated playoff record at home. The Celtics had defeated the #8 seeded Atlanta Hawks 4–3 and then defeated the #4 seeded Cleveland Cavaliers 4–3 as well. They advanced to the 2008 NBA Finals after a 4–2 Conference Finals series win over the Detroit Pistons.
On June 12, 2008, Allen would play all 48 minutes and contribute greatly with 19 points and 9 rebounds in what would become the largest comeback in NBA Finals history, with the Celtics coming back from a 24-point deficit at one point in the second quarter and a deficit as large as 20 in the third. In that game, Allen would make the game clinching layup with 16.4 seconds remaining to give the Celtics a 5-point lead and put the game away.
On June 17, 2008, in the series-ending Game 6 of the NBA Finals, Allen tied an NBA Finals record with seven three-pointers in the Celtics' 131–92 victory over the Los Angeles Lakers. With the victory, Allen secured his first NBA championship.
On February 5, 2009, Allen was named as the All-Star replacement for Orlando Magic point guard Jameer Nelson. This marks Allen's ninth time on the All-Star team and the second straight year he has made it alongside teammates Kevin Garnett and Paul Pierce.
On February 22, 2009, with his streak ending at 72, Allen broke the Celtics franchise record in consecutive free throws made previously set by Larry Bird (71).
In the first round of the 2009 playoffs against the Chicago Bulls, game one proved to be tough for Allen. He ended up shooting 1–11 from the field, and the Celtics lost by two. In game 2 however, Allen nailed the game winning three-pointer at the end of regulation with two seconds left to cap off an impressive 30 point night. In Game 6, despite a Boston loss, Allen managed to score 51 points. Boston ended up winning the series in 7 Games. He reunited with former teammate and co-captain of the Seattle Sonics, Rashard Lewis in the Conference Semifinals, this time as opponents, however. After 4 games and a 2–2 series tie, his highest scoring game was a 22-point performance in a Game 2 victory. The Celtics went on to lose in seven games as they failed to defend their title.
On December 10, 2009, Allen scored 18 points to reach the 20,000 point total for his career.
On June 6, 2010, in Game 2 of the 2010 NBA Finals in a 103–94 victory against the Los Angeles Lakers, Allen set the record for most three-pointers made in an NBA Finals game on 8–11 shooting to tie the series 1–1. He broke Michael Jordan's record of six three-pointers in one half by hitting seven in the first half and had tied Scottie Pippen and Kenny Smith's record for most three-pointers in one game in the NBA Finals by half time. Allen also became the first player in NBA history to have two separate games of scoring at least seven three-pointers in the NBA Finals. The Celtics would win two of three games in Boston, before losing the last two games and the series in Los Angeles.
After becoming a free agent on July 1, 2010, Allen re-signed a two year-deal with the Boston Celtics on July 7 worth $20 million.
During a game against the Los Angeles Lakers on February 10, 2011, Allen became the all-time NBA leader in total 3-point field goals made (2,562), surpassing Reggie Miller's record of 2,560. The 35-year-old shooting guard finished the game with 20 points but the Celtics eventually lost 92–86. In the 2010–11 NBA season Allen was named to his tenth All-Star game alongside teammates Rajon Rondo, Paul Pierce and Kevin Garnett. Also during All-Star Weekend, Allen competed in the Three-Point Shootout, but lost to James Jones of the Miami Heat.
Allen rejected a two-year, $12 million offer to return to the Boston Celtics and accepted a three-year deal with the Miami Heat, who were limited to their mid-level exception amount of slightly more than $3 million per season. During Allen's first season with the Miami Heat, he averaged 10.9 points per game and made 88.6% of his free throws while playing an average of 25.8 minutes per game. On April 25, 2013, while playing Milwaukee in Game 3 of the first round of the playoffs, Allen made his 322nd career three-pointer in the playoffs, which broke Reggie Miller's record for most three-pointers made in NBA playoff history. Allen returned to his favorite #34 jersey upon joining the Heat.
In Game 6 of the NBA Finals against the San Antonio Spurs, as the Heat trailed by three points, Allen made a game-tying 3-pointer with 5.2 seconds left in the fourth quarter. Having rallied from a 10-point deficit at the end of the third quarter, the Heat won in overtime 103–100 over the San Antonio Spurs and forced a seventh game in the series. With the Heat leading 101–100, Allen stole the ball from Manu Ginóbili under the Spurs' basket and drew a foul with 1.9 seconds remaining in the overtime period. Allen made both ensuing free throws to put Miami up 103–100. Allen played 41 minutes in Game 6 off the bench and scored 9 points on 3-for-8 field goal shooting; his game-tying three-pointer was the only three-pointer he made in three attempts. Allen also had one defensive rebound and two assists. In Game 7, which the Heat won 95–88 to win its second consecutive championship, Allen played 20 minutes off the bench. Although he made none of 4 field goal attempts and had 3 turnovers, Allen recorded 4 defensive rebounds and 4 assists.
On June 29, 2013, Allen picked up his $3.23 million player option to stay with the Heat through the 2013–14 NBA season. During the regular season, Allen played in 73 games, starting 9 of them, averaging 9.6 points and shooting 37 percent from three point range. In the playoffs, Allen scored 19 points on 4–7 three point shooting in game 4 of the conference semifinals against the Brooklyn Nets. Then in Game 3 against the Indiana Pacers in an Eastern Conference Finals rematch, Allen hit four 3-point shots in the fourth quarter and put them up 2–1 in the series. The Heat won the series in six games to advance to the NBA Finals for the fourth straight year and Allen's second. The Heat faced the Spurs again in the 2014 NBA Finals, but they lost the series in five games.
Allen became a free agent in the 2014 off-season, and after much anticipation of a return at some point during the 2014–15 season, he announced his decision to sit out the season and focus on 2015–16. However, he did not end up joining a team for the 2015–16 season. In July 2016, Allen began mulling a comeback, and admitted he was "intrigued" by the possibility of competing for another championship. The following month, Allen had talks with the Boston Celtics and the Milwaukee Bucks regarding a possible return, but ultimately decided to officially retire from the game on November 1, 2016.
In 1998, Allen co-starred alongside Denzel Washington in the Spike Lee film He Got Game as high school basketball phenomenon Jesus Shuttlesworth. Roger Ebert praised his performance and said Allen "is that rarity: an athlete who can act," while New York magazine described him as "graceful and fast in the basketball scenes" while giving "a somberly effective minimalist performance." His role as Jesus Shuttlesworth earned him that name as a nickname from teammates and fans. Allen also appeared as Marcus Blake in the 2001 film Harvard Man. In 2015, Allen co-starred alongside Kyrie Irving, Baron Davis and J. B. Smoove as Walt in the fourth episode of "Uncle Drew", a series of Pepsi Max advertisements written and directed by Irving.
Allen married singer/actress Shannon Walker Williams on August 16, 2008 after having dated since 2004. They have four children together.
Allen says that he has a borderline case of obsessive–compulsive disorder, to which he attributes his consummate shooting style. He is a member of the Air Jordan brand, which is a subsidiary of Nike.
Allen served as NBA Spokesperson for the Jr. NBA / Jr. WNBA program and was selected as NBA Spokesman for the Thurgood Marshall College Fund. He also started the "Ray of Hope" Foundation to assist charities in several communities.2× NBA Champion: 2008, 2013
3× Gold medalist as member of the USA men's basketball team: 1995 Summer Universiade, 2000 Summer Olympics & 2003 FIBA Americas Championship.
10× NBA All-Star: 2000–2002, 2004–2009, 2011
All-NBA Second Team: 2005
All-NBA Third Team: 2001
NBA All-Rookie Second Team: 1997
NBA Joe Dumars NBA Sportsmanship Award: 2003
The Sporting News "Good Guy": 2000, 2001, 2005
NBA All-Star Weekend Three-point Shootout champion: 2001
USA Basketball Male Athlete of the Year (1995)
3-point field goals made, career: 2,973
3-point field goal attempts, career: 7,429
3-point field goals made, career: 385
3-point field goals attempted, game: 18, Boston Celtics at Chicago Bulls, April 30, 2009 (3 OT)
Tied with Stephen Curry (April 23, 2015), Damian Lillard (May 9, 2016), Klay Thompson (May 28, 2016), and Russell Westbrook (April 25, 2017)
3-point field goals made, game: 8, Boston Celtics at Los Angeles Lakers, June 6, 2010
3-point field goals made, half: 7, first half, Boston Celtics at Los Angeles Lakers, June 6, 2010
3-point field goals made, quarter: 5, second quarter, Boston Celtics at Los Angeles Lakers, June 6, 2010
3-point field goal attempts, half: 7, second half (2005)
Consecutive games played: 400, from 11/1/96 to 12/20/01
3-point field goals made, career: 1,051
3-point field goals made, season: 229 (2001–02)
3-point field goals made, game: 10, vs. Charlotte Hornets, April 14, 2002
3-point field goals made, half: 8, second half, vs. Charlotte Hornets, April 14, 2002
Tied with Tim Thomas (second half, at Portland Trail Blazers, January 5, 2001) and Michael Redd (second half, vs. Houston Rockets, February 20, 2002)
3-point field goal attempts, season: 528 (2001–02)
3-point field goal attempts, game: 17, at Cleveland Cavaliers, December 9, 2002 (2 OT)
3-point field goal attempts, game (regulation): 14, four times (tied with Michael Redd)
14, vs. Utah Jazz, April 12, 2001
14, vs. New York Knicks, December 4, 2001
14, vs. Charlotte Hornets, February 18, 2002
14, vs. Charlotte Hornets, April 14, 2002
3-point field goals made, season: 269 (2005–06)
3-point field goal attempts, season: 653 (2005–06)
Highest free throw percentage, season: .952 (237/249) (2008–09)
Consecutive free throws made: 72, from December 25, 2008 to February 23, 2009
Highest 3-point field goal percentage, 2011–12 season: .453 (106/234).