At only 18 years of age, Al was selected by the Pacers with the 25th pick of 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 (with whom he would later be traded to Golden State). Harrington finally became a nightly starter, but the Hawks did not fare as successfully as the Pacers did after his departure.
On the eve of the 2008–2009 season, Harrington told coach Don Nelson that he wanted to be traded. Harrington, now with agent Dan Fegan, stopped playing after November 5, 2008, claiming to have a sore back. On November 21, Harrington was traded to the New York Knicks for guard Jamal Crawford. The same day, Harrington said he expected to pass his physical and to begin playing for the Knicks by early in the next week. Al Harrington and Stephen Jackson came out with a new footwear line called Protege.
On July 15, 2010, Al Harrington signed a contract with the Denver Nuggets. During the 2010-2011 season, he averaged 10.5 ppg, 4.5 rpg, and 1.4 apg in 22.8 mpg. The Denver Nuggets finished 50-32, 5th best in the Western Conference and 2nd in the Northwest Division. The Oklahoma City Thunder defeated the Denver 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, averaging 5.1 ppg, 2.7 rpg, and 1.0 apg in 11.7 mpg. The Magic finished with a 20-62 record, the worst record in the NBA. On August 2, 2013, Harrington was waived by the Magic.
^"Al Harrington traded for Stephen Jackson", Inside Hoops, July 15, 2004. Accessed June 4, 2008. "A 6-9 forward from Orange, New Jersey, Harrington prepped at St. Patrick's High in Elizabeth, New Jersey and he was the first player ever drafted from the high school ranks by the Pacers."