Posey drove in six runs, going 4 for 4 and scoring three times. He capped a seven-run fourth inning with his slam.
Andres Torres, Aubrey Huff and Posey all homered in the first -- 1999 was the last time the Giants connected three times in the opening inning.
Lincecum (9-4) needed only a fraction of the support. The two-time reigning NL Cy Young Award winner scattered four hits in seven innings.
Lincecum had lost his previous two starts. He avoided dropping three starts in a row for the first time in his four-year career.
The Giants are a win away from completing a four-game sweep on Thursday after coming into this series as losers of eight of nine. San Francisco has outscored the Brewers 27-4 in the first three games and every player in its starting lineup had a hit Wednesday night.
Lincecum, the first Giants pitcher with three straight All-Star selections since Juan Marichal, had lost consecutive starts five times in his career. Staked a 4-0 lead when he took the mound, he stayed aggressive and kept the Brewers guessing for his 49th career win.
Torres led off the game with a homer and Huff made it 2-0 against Chris Narveson (7-6) with a second-deck shot. Posey's two-run homer made it 4-0.
Posey moved from his usual catcher's spot to first base a day after being hit by a pitch on the left knee. He has 13 hits in his last 25 at-bats, and saved his biggest shot for the fourth.
Narveson's throwing error on a bunt by Lincecum helped the Giants loaded the bases and Freddy Sanchez tripled to make it 7-1. Narveson walked two more to load the bases again and Posey homered off reliever Chris Capuano.