The Falcons also cut veteran quarterback Byron Leftwich last month.
The release of Harrington may increase the chances the team will take a quarterback, perhaps Matt Ryan, with the No. 3 overall selection in the draft.
Ryan, from Boston College, is widely considered the top quarterback in the draft.
The Falcons re-signed Chris Redman to a two-year deal last week, but the only other quarterbacks on the roster are D.J. Shockley and Michael Vick. Shockley has never played in a regular-season game and Vick is serving a 23-month prison sentence after pleading guilty to federal charges in a dogfighting operation.
Vick also is serving an indefinite suspension imposed by NFL commissioner Roger Goodell.
Shockley, a 2006 seventh-round pick from Georgia, missed last season with a knee injury.
The 29-year-old Harrington played in 12 games with 10 starts last season. He completed 215 of 348 passes for 2,215 yards with seven touchdowns and eight interceptions. He set a career high by completing 61.8 percent of his passes, but the Falcons were only 3-7 in his starts.
Detroit drafted Harrington No. 3 overall in 2002.
Following five seasons with the Lions and one year in Miami, Harrington signed a two-year, $6 million contract to become Vick's backup. He moved into the starting role when Vick's legal problems began.
Harrington might have remained to serve as the mentor for a young quarterback, but Redman has assumed the role of veteran stand-in.
"I just want to contribute however I can, whether it's being a starter or however I can to be a part of the team," Redman said Monday.
After going three years without taking a snap in the NFL, Redman finished last season as the starter. His unlikely revival was capped by a four-touchdown game against Seattle in the final game of the season. He was selected the NFC offensive player of the week.
The release of Harrington came only two days after the team cut popular running back Warrick Dunn.
Four-time Pro Bowl tight end Alge Crumpler, offensive tackle Wayne Gandy, defensive tackle Rod Coleman and Leftwich were among the players released last month as new general manager Thomas Dimitroff and coach Mike Smith began remaking the roster.
The Falcons already have been active in free agency, signing running back Michael Turner to a six-year deal this week. Dimitroff also added safety Erik Coleman, cornerback Von Hutchins and tight end Ben Hartsock in the first two days of the free-agent period.
The signing of Turner, who will share carries with Jerious Norwood, makes it less likely the team will use the first-round pick on a running back such as Darren McFadden of Arkansas.
"Having two talented running backs here presents a different dynamic than a couple days ago," Dimitroff said after signing Turner. "With that said, I'd prefer not to comment on the direction we're going to take in the draft."