On Monday, he gave the A's 7 2/3 innings, yielding two runs, four hits and three walks with five strikeouts, becoming just the third Oakland starter this season to pitch into the eighth.
For much of the night, it looked like he would complete it, too, finishing the seventh at just 82 pitches with only three baserunners allowed. But he tired quickly thereafter, offering up consecutive base hits and a walk to load the bases. Mengden got a double-play ball from Chris Stratton, but his ensuing free pass to Joe Panik prompted manager Bob Melvin from the dugout.
"Got a little bit tired at the end there, but we've basically seen the same guy," Melvin said. "He's efficient, throws strikes with all four pitches, keeps you off balance, confident in what's he doing. It's been really fun to watch."
"I thought he looked really good," Giants first baseman Brandon Belt said. "He has good stuff and was able to locate it. I'd say he kind of kicked our butts, to be honest with you. He was able to do pretty much whatever he wanted to do all night."
Mengden, who was only the third pitcher in Oakland history to lose each of the first three games of his career, finally received the kind of run support that eluded him in those outings -- though he hardly needed much of it.
The A's scored eight times on his watch, after totaling only two in his previous 18 innings of big league work. Shortstop Marcus Semien was responsible for four of them Monday, launching a three-run home run that highlighted a five-run second inning and tacking on an RBI double in the sixth.
"This was his best outing," Semien said, "but he's given us a chance every time. It's good that we came in with some hot bats and that's what got him the win.
"He's come up here and shown that he belongs. He just has to stay confident and keep working hard and good things will happen."
Jane Lee has covered the A's for MLB.com since 2010.