Yoervis Medina (1-1) was dealt the loss after he allowed consecutive singles to open up the 10th and was forced to walk Chase Headley to load the bases. Venable ripped an inside pitch just off Seattle first baseman Kendrys Morales' glove, scoring Chris Denorfia for the game-winner.
"I got a decent pitch to hit and was able to find a spot where they couldn't make a play," Venable said of the game-winning hit.
Mariners reliever Tom Wilhelmsen blew the save in the ninth after giving up Kyle Blanks' game-tying sacrifice fly.
Headley added a solo homer for the Padres, who have won two straight after dropping the first of two games in Seattle. Luke Gregerson (3-2) earned the win, tossing a perfect 10th inning.
Jason Bay hit a tie-breaking homer in the top of the ninth and Morales drove in another run for the Mariners, losers in seven straight road contests.
"We have got to get the offense going," Mariners manager Eric Wedge said. "We're not scoring enough runs and we're inconsistent. The guys have to step up."
Wilhelmsen took the hill in the ninth holding a 2-1 lead. The inning got off to a rocky start, as Yasmani Grandal walked and Jedd Gyorko singled. After both runners moved up one base thanks to past ball, Yonder Alonso was intentionally walked to load the bases.
Blanks came through with a deep fly to center, tying the game at 2-2. Wilhelmsen battled and got out of the inning by forcing Alexi Amarista to pop out and Carlos Quentin to groundout to third and end the damage.
Huston Street relieved San Diego starter Eric Stults in the ninth, but Bay greeted him with a leadoff shot to left to give the Mariners a 2-1 lead. Street fell behind in the count, 2-0, while Bay's homer barely cleared the left field wall, and the grasps of Blanks.
Headley put the hosts on the board with two outs in the first, clubbing Seattle starter Joe Saunders' offering into the second deck of the left-field bleachers for a 1-0 lead.
Stults easily had one of his best starts if the season, allowing one run on three hits with a season-high 12 strikeouts over eight frames. The lefty cruised through the first three innings and retired the first 10 batters he faced, while striking out six along the way.
His early success came to an end in the fourth.
With one out, Eric Chavez and Kyle Seager hit a single and a double, respectively, to put runners on second and third. Morales then tied the game with a run-scoring groundout to second.
Stults and Saunders continued to dual over the next three-plus innings with both offenses struggling to find much success.
Carter Capps relieved Saunders in the eighth, but found himself in some trouble.
Amarista smacked a one-out double and Capps then issued a walk to Mark Kotsay. Denorfia beat out a high-chopper to third to load the bases, though Capps come through in the clutch by fanning Everth Cabrera and forcing Headley to pop out, ending the threat.
Seattle second baseman Nick Franklin recorded his first career MLB hit with a single in the fifth ... Saunders, who lasted seven innings and allowed just one run on four hits, is still 0-4 in six road starts this season ... Seattle's Michael Morse missed the game after suffering a right quadriceps strain Tuesday ... Saunders has allowed one earned run over his last 29 2/3 innings versus the Padres.
05/30 02:46:28 ET
Tuesday, December 17th, 2013 - Tuesday, January 7th, 2014
Sunday, December 22nd, 2013
Monday, December 23rd, 2013