Collmenter pitched six innings and drove in a run during Arizona's six-run second and the Diamondbacks beat the reeling New York Mets 11-5.
"He kept his composure and we needed innings," Diamondbacks manager Kirk Gibson said. "A lot of guys would have lost their composure there. The guy's got ice in his veins."
Willie Bloomquist had two RBI singles for the Diamondbacks, who have won six of seven.
Collmenter (3-2), who allowed five runs, seven hits and a walk with five strikeouts, got stronger as the game continued. He retired the final seven batters he faced.
"He made two bad pitches but he's a warrior," catcher Henry Blanco said. "He kept guys off balance, kept them off the bases and gave us a chance to get the win."
Jonathon Niese (7-5) turned out to be his own worst enemy. After loading the bases in the second with a two-run lead, he walked Blanco to cut New York's lead to 2-1 and Collmenter singled to tie the game.
The walk to Blanco was Niese's first in his past 25 innings.
"It's a pitcher's worst nightmare to walk a guy with the bases loaded," Niese said.
But the nightmare got worse.
Bloomquist topped a ground ball near the third-base line. Niese sprinted off the mound to corral the ball, spun and tried to fire the ball to first but his throw bounced once in the grass yards short of the bag and rolled down the right-field line as all three runners scored.
"I think everyone in the ballpark, including myself, know that I shouldn't have thrown that ball," Niese said. "When you are in the heat of the moment, when you are out there battling, I just tried to force it when I shouldn't have. In hindsight, I wish I would have done a lot of things different."
Mets manager Terry Collins said Niese was trying to do a little too much to stop the Arizona rally.
"I'm sure he wishes he could take it back," Collins said. "He didn't have a play on it. If (third baseman) Wright doesn't get to it, it's pretty much a base hit. It was just one of the mistakes we made tonight."
Collmenter scored on a sacrifice fly by Aaron Hill. Left fielder Jason Bay finally ended the inning when he tumbled head first into the stands in foul territory as he caught Jason Kubel's high fly ball.
Niese allowed eight runs -- six earned -- and nine hits with five strikeouts and a walk over six innings.
"One of the reasons I left him in is I wanted him to work a little bit," Collins said. "I wanted to see if he could get himself into a little bit of a feel, a rhythm, get his release point back and get into synch so he could leave feeling better about himself than he did after the second inning."
Bloomquist hit his second run-scoring single in the fourth and Paul Goldschmidt singled home Kubel in the fifth to stretch Arizona's lead to 8-5. Justin Upton added an RBI single and Chris Young lined a two-out double to left in the seventh against Elvin Ramirez to make it 11-5.
"It looked like it was going to be a long night the way it was looking but Josh strapped it down," Young said. "We were manufacturing runs and just trying to keep the pressure on them."
The teams had played seven straight games won by two runs or less since Aug. 12, 2011. ... The Diamondbacks have scored six or more runs in an inning four times this season. ... Arizona had scored 10 runs in its four previous games against the Mets combined. . Davis has hit 12 of his team-high 17 homers on the road. ... Bay finished 0-for-4 and is hitless in his last 15 at-bats. He is 4-for-30 since returning from the DL on July 17. ... Niese had won five of his past seven decisions. ... Mets closer Frank Francisco (oblique), on a minor league rehab assignment, earned the save for Double-A Binghamton in a 2-1 win against Portland in the first game of a doubleheader. Francisco allowed one hit and no runs while throwing 16 of his 21 pitches for strikes. ... Mets OF Mike Baxter (collarbone, rib) went 1-for-4 with a single for Triple-A Buffalo at Louisville. He played nine innings in left field. ... RHP Chris Young takes the mound on Saturday night for the Mets against RHP Ian Kennedy. Young pitched a season-high seven innings but took a no-decision in New York's 8-2 loss to Washington on July 23. He has not faced Arizona since April 6, 2010 as a member of the San Diego Padres. Kennedy will be making his 100th career appearance.