Forsythe, activated from the 60-day disabled list on Sunday, hammered Steve Edlefsen's 1-0 sinker into the left field bleachers to lift the Padres to their third win in 14 games.
"As a starter your job is to put your team in a position to win. Obviously, I haven't been doing that up until now and I'm definitely going to be making that my focal point for the next however many months we have left," said Lincecum.
Lincecum was in line for the victory after Theriot drove in Brandon Crawford with an opposite-field single in the seventh inning.
Quentin, also recently off the DL, knotted things at 5-5 by golfing his fifth home run to straightaway center.
The Giants had been 26-0 when leading after seven innings.
The Padres scored four runs off Lincecum in the fourth, which began with Quentin skying one into the second deck in left field.
"That ball was crushed," Padres manager Bud Black said of Quentin's home run. "That's what we envisioned. We figured he would be a force in the lineup."
Theriot started San Francisco's comeback in the third with an RBI single, then doubled home Gregor Blanco to jump-start a three-run sixth. Posey and Brandon Belt each knocked in runs with sacrifice flies.
Street was also activated off the DL on Tuesday...Quentin has 15 career multi- home run games...Theriot had four RBI in 29 games coming in...Lincecum struck out eight over six innings and was charged with four runs on five hits and eight walks...San Diego starter Anthony Bass yielded four runs on seven hits in 5 1/3 frames...Pagan went 2-for-4 to extend his hitting streak to 13 games...Blanco had three of the Giants' 13 hits.
For more on the Giants, head over to McCovey Chronicles.