The 29-year-old Harrington became to a starter last season when Michael Vick was suspended after his legal troubles from dog-fighting activities.
Harrington was the third overall pick in the 2002 NFL draft by Detroit.
While playing for the Lions, Falcons and Miami Dolphins in five NFL seasons, the University of Oregon product has started 55 of 58 games, passed for 10,242 yards, and compiled a 56.1 completion percentage with 60 touchdown passes but 62 interceptions.
His release follows the earlier release of Byron Leftwich, and with the inexperience of Chris Redman, leaves the door open for the Falcons to possibly make Boston College quarterback Matt Ryan their top pick in this year's draft next month.
Also in their off-season housecleaning of older players, the Falcons have released tight end Alge Crumpler, left offensive tackle Wayne Gandy, defensive tackle Rod Coleman and running back Warrick Dunn.