Craig Counsell started the 12th with a single before Ashby (0-1) retired the next two batters. That set the stage for Overbay, who batted for pitcher Matt Mantei.
Just 2-for-15 in his rookie season, Overbay drove Ashby's first pitch over the wall in right-center to give the Diamondbacks just their second win in seven games.
"When you are 2-for-15 and you haven't hit too many balls hard, it is kind of frustrating," Overbay said. "He threw the sinker, which is his bread and butter. Nine times out of 10, he gets you to roll over on it. I just wanted it to get it in the gap. When I saw the centerfielder and rightfielder look up, I thought this might have a chance. It is a big confidence boost for me, just to hit the ball hard."
Mantei (2-0) picked up his second win, despite allowing the tying run in the bottom of the 10th. He gave up two hits in three innings, and Scott Service retired the side in order in the 12th for his first save Sept. 22, 2000.