Allen was a ten-time NBA All-Star, and won an Olympic gold medal as a member of the 2000 United States men's basketball team.

Allen played college basketball for the Connecticut Huskies for three seasons, before he was selected with the fifth overall pick in the 1996 NBA draft, and developed into a prolific scorer for the Milwaukee Bucks, featuring alongside Glenn Robinson and Sam Cassell as the team achieved playoff success.

In 2003, he was traded to the Seattle SuperSonics, where he solidified his reputation as a scorer, breaking several league records for three-point and free throw shooting.

Allen was later traded to the Boston Celtics, where he formed a "Big Three" with Kevin Garnett and Paul Pierce; the team won an NBA championship in 2008.

[4] A military child, he spent time growing up in Saxmundham, Suffolk, England, in Altus, Oklahoma, at Edwards Air Force Base[4] in California, and in Germany.

[6] Fueled by his desire to become the top player on his military base, Allen practiced at length daily, so long as it did not interfere with his studies.

[6] 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.

Immediately after his selection, Allen and Andrew Lang were traded to the Milwaukee Bucks for the rights to fourth pick Stephon Marbury.

[17] 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.

[18] 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.

On the brink of missing the playoffs for the second straight year, on April 18, 1998, the last game of the regular season, Allen tallied a 40-point double-double with 10 rebounds.

[20] On February 10, 1999, following the 1998–99 NBA lockout, Allen signed a six-year, $70.9 million extension with the Bucks, making him the highest paid player in franchise history at that point.

[30] Allen remained with the Bucks midway through the 2002–03 season, when he was dealt to the Seattle SuperSonics, along with Ronald Murray, former UConn teammate Kevin Ollie, and a conditional first round draft pick, in exchange for Gary Payton and Desmond Mason.

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.

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.

This marked Allen's ninth time on the All-Star team and the second straight year he had 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).

[46] During a game against the Los Angeles Lakers on February 10, 2011, Allen became the all-time NBA leader in total three-pointers made (2,562), surpassing Reggie Miller's record of 2,560.

[47] In the 2010–11 NBA season Allen was named to his tenth All-Star game alongside teammates Rajon Rondo, Paul Pierce and Kevin Garnett.

[48] During the 2011–12 NBA season, Allen was privately frustrated with trade rumors in February, issues with teammate Rajon Rondo, and an ankle injury that ultimately required surgery in June 2012 to remove bone spurs.

Further internal discord with a bench role and a Rondo-focused approach led to Allen rejecting the Celtics and instead choosing to sign a two-year, $6 million deal with the Miami Heat.

[36] 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 to force overtime.

[53] 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.

[54] 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.

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.

[60] The following month, Allen had talks with the Boston Celtics and the Milwaukee Bucks regarding a possible return,[61] but ultimately decided to officially retire on November 1, 2016.

[66] In 1998, Allen co-starred alongside Denzel Washington in the Spike Lee film He Got Game as high school basketball phenomenon Jesus Shuttlesworth.

B. Smoove as Walt in the fourth episode of "Uncle Drew", a series of Pepsi Max advertisements written and directed by Irving.