With the score tied 2-2, Everth Cabrera led off the seventh with a single off Adam Ottavino (2-1). After Matt Reynolds came in to pitch, John Baker sacrificed Cabrera to second.
Venable followed by lining a 2-1 pitch from Reynolds into right field for a solid hit and Cabrera scored easily, putting San Diego ahead for good.
Dale Thayer (2-2) earned the victory with 1 1/3 scoreless innings in relief of starter Ross Ohlendorf and Huston Street racked up his 16th save as the Padres took the rubber match of a three-game series.
Rockies starter Christian Friedrich held the Padres to two runs on three hits over five innings.
DJ LeMahieu went 2-for-3 with an RBI double for Colorado.