[1] Allen attended Crane High School in Chicago, where he played alongside fellow future NBA player Will Bynum.

[2] For his junior season, Allen transferred to Oklahoma State University, where he averaged 14.4 points and 5.4 rebounds in 32 games (28 starts) in 2002–03.

He was subsequently named Big 12 Player of the Year and earned an Honorable Mention All-America by the Associated Press.

On March 20, 2006, Allen recorded a season-high 18 points and 4 steals in a home loss to the Los Angeles Lakers.

Eight days later, he broke his season-high as he scored 23 points to go along with 4 rebounds and 3 steals in a home game loss to the Cleveland Cavaliers.

Allen went on to average 7.2 points, 2.2 rebounds, 1.3 assists, and 1.0 steal in 19.2 minutes in 51 games (nine starts) in his sophomore season with the Celtics.

In a contest against the Indiana Pacers on January 10, 2007, Allen suffered a season-ending tear of the anterior cruciate ligament (ACL) and medial collateral ligament (MCL) of his left knee;[2] Allen hurt the knee when he landed awkwardly after an uncontested dunk attempt after the whistle in the final minutes of the 97–84 loss.

In his first game after the injury, Allen scored 4 points and grabbed 5 rebounds during a victory over the visiting Washington Wizards.

In his first double-digit scoring output since the injury, Allen recorded 13 points in a win over the New Jersey Nets on November 14.

[2] In July 2008, Allen re-signed with the Celtics after agreeing to a two-year guaranteed deal paying $2.5 million each year.

[2] Allen played a pivotal defensive role for the Eastern Conference champion Celtics during the 2009–10 season, averaging 6.1 points, 2.7 rebounds, 1.3 assists, and 1.07 steals in 16.5 minutes in 77 games (eight starts).

[12][13] Allen concluded his fourth season in Memphis and 10th in the NBA with averages of 9.0 points, 3.8 rebounds, and 1.64 steals on .494 shooting in 23.2 minutes in 55 games (28 starts).

Allen recorded 10 offensive rebounds in Game 4 against the Thunder, becoming the sixth reserve to record 10+ offensive rebounds in a postseason game since the 1986 Playoffs, joining centers John Salley, Roy Tarpley, Jayson Williams, Greg Ostertag, and Jeff Foster.

[22] Standing at 6 feet 4 inches tall (1.93 m) and weighing 213 pounds (97 kg), Allen played both the shooting guard and small forward positions.

[24] On February 26, 2016, in a game against the Los Angeles Lakers, Kobe Bryant presented Allen with an autographed pair of shoes that read, "To Tony, the best defender I ever faced!"

He has two sisters, Ebony and Dominique, and a brother, Ryan, who played on the Grizzlies' 2012 Summer League squad following a four-year collegiate career at Wisconsin-Milwaukee.

[2] In November 2005, Allen was indicted on three counts of aggravated battery following a fight which led to a shooting in August 2005 in Chicago.

[29] On October 7, 2021, Allen was indicted for insurance fraud in the Southern District of New York for allegedly defrauding the NBA's health and welfare benefit plan.

[30] On August 8, 2023, Allen was sentenced to three years of probation and community service with supervision, avoiding prison altogether, due to him paying back most of the $420,000 he had taken illegally before being arrested.

[31][32] The arrest in 2021 was considered the biggest reason for why Allen's #9 wasn't retired during the 2021–22 season alongside teammate Zach Randolph and instead was delayed until March 15, 2025, against the Miami Heat.