Harrington is a cousin of NBA player Dahntay Jones.
Born in Orange, New Jersey, Harrington grew up in Roselle, New Jersey and played high school basketball at St. Patrick High School in Elizabeth, New Jersey. He was named both Gatorade and USA Today's National Player of the Year, as well as a 1998 McDonald's High School All-American after his senior season.
At only 18 years of age, Harrington was selected by the Indiana Pacers with the 25th overall pick in the 1998 NBA draft, and spent six seasons with them, primarily coming off the bench. Harrington really began to come into his own in the 2001–02 season, in which he averaged 13.1 points and 6.3 rebounds per game, but his season came to an end in a game against the Boston Celtics when he suffered a knee injury that forced him to miss the final 38 games of the season.
He made a comeback in the 2002–03 season, becoming the only Pacer to play in all 82 games that year. He averaged 12.2 points and 6.0 rebounds per game while starting in 37 games. The following season, he boosted his averages slightly to 13.3 points and 6.4 rebounds per game, and finished second in voting for the NBA Sixth Man of the Year Award. He was an integral part of the Pacers' first run to the Eastern Conference Finals since 2000.
On July 15, 2004, Harrington was traded to the Atlanta Hawks in exchange for Stephen Jackson. Harrington finally became a nightly starter, but the Hawks did not fare as successfully as the Pacers did after his departure.
On August 22, 2006, Harrington and John Edwards were acquired by the Indiana Pacers from the Atlanta Hawks in a sign-and-trade deal that also sent the Hawks a 2007 first round draft pick. In 2006–07, Harrington wore jersey #32 because his first choice #3 was worn by teammate Šarūnas Jasikevičius (saying it stands for "number three, and it's my second time around").
On January 17, 2007, Harrington was dealt to the Golden State Warriors along with teammates Stephen Jackson, Šarūnas Jasikevičius, and Josh Powell for Troy Murphy, Mike Dunleavy, Ike Diogu, and Keith McLeod.
On November 21, 2008, Harrington was traded to the New York Knicks in exchange for Jamal Crawford. In his two seasons with the Knicks, he played the best basketball of his career, but did not reach the play-offs in either season. In 140 games (66 starts), he averaged 19.2 points, 5.9 rebounds, 1.4 assists and 1.0 steals in 32.7 minutes per game.
On July 15, 2010, Harrington signed a multi-year deal with the Denver Nuggets. During the 2010–11 season, he averaged 10.5 points, 4.5 rebounds, and 1.4 assists in 22.8 minutes per game. The Nuggets finished 50-32, 5th best in the Western Conference and 2nd in the Northwest Division. The Oklahoma City Thunder defeated the Nuggets in five games in the first round of the 2011 Playoffs.
On August 10, 2012, Harrington was traded to the Orlando Magic in a four-team trade which sent Dwight Howard to the Los Angeles Lakers. He only played 10 games for the Magic in 2012–13, averaging 5.1 points, 2.7 rebounds, and 1.0 assists in 11.7 minutes per game. The Magic finished with a 20-62 record, the worst record in the NBA. On August 2, 2013, Harrington was waived by the Magic.
On August 14, 2013, Harrington signed with the Washington Wizards. He came off the bench in all 37 games he played for Washington, averaging 6.6 points and 2.4 rebounds per contest. The Wizards made their first playoff appearance since the 2008, and Harrington played in seven of the teams 11 postseason games.
On August 11, 2014, Harrington signed with the Fujian Sturgeons of the Chinese Basketball Association. On November 25, 2014, he parted ways with Fujian amidst NBA interest.
After returning to the United States and not receiving any NBA offers, Harrington announced his retirement from professional basketball on March 18, 2015 after averaging 13.5 points and 5.6 rebounds in a 16-year career that included stops with seven teams.
Harrington later came out of retirement and signed with the Sydney Kings of the National Basketball League on October 28, 2015 as an injury replacement for Josh Childress. He made his debut for the Kings two days later, scoring 12 points off the bench in an 87–78 win over the Townsville Crocodiles. With Childress set to return from injury, Harrington played his last game with Sydney on November 19, recording 18 points and 6 rebounds in a loss to the New Zealand Breakers. In six games for the Kings, he averaged 17.7 points, 6.8 rebounds and 2.7 assists per game.
In the summer of 2017, Harrington competed in the inaugural season of the Big3 basketball league. He played for Trilogy and served as co-captain with former teammate Kenyon Martin. The team went an undefeated 10-0 that season, winning the first-ever Big3 Championship.