Zito (11-8) silenced the Dodgers on four hits over 6 1/3 scoreless innings, walking three and striking out four.
The Giants took two of three in the crucial series against the second-place Dodgers, increasing their lead over them to 5 1/2 games with 22 contests remaining in the season.
Hunter Pence provided a two-run double in the first inning and Buster Posey socked a solo homer for the Giants.
Joe Blanton (9-13) was hung with the loss for Los Angeles, yielding four runs on six hits in a 5 1/3-inning effort.
The Dodgers also fell 1 1/2 games behind St. Louis in the National League wild card race.