Ryan drove the Falcons 28 yards for a 10-3 lead after Detroit's Stefan Logan fumbled a first-quarter kickoff return, capping the drive with a 1-yard TD plunge.
The signal-caller (20-of-34, 218 yards) made it 17-6 with an 18-yard scoring strike to Roddy White 25 seconds before halftime, and the Falcons (4-3) held on to win their second straight.
Michael Turner picked up 122 yards on 27 carries for the victors.
The Lions (5-2), meanwhile, dropped their second consecutive game after opening the season with five straight wins.
Matthew Stafford managed only 15-of-32 passing efficiency for 183 yards and a touchdown as Detroit went 1-for-12 on third-down plays.
Calvin Johnson hauled down a 57-yard TD pass among his five receptions for 115 yards in the loss.