Chris Narveson, on the other hand, needed a stellar effort over nearly seven innings to reach his goal.
McGehee got to 100 RBIs on one of several sloppy plays by the Mets and Hart reached 100 RBIs two batters later, driving in Braun for his 100th run scored, as the Milwaukee Brewers beat New York 9-2 Thursday night.
By allowing just one run over 6 2/3 innings, Narveson (12-9) lowered his ERA to under 5.00 -- he left with it right at 4.99.
"It was pretty sweet," he said. "It was a goal of mine to get it in the last month."
McGehee was stuck on 99 RBIs for seven games before getting his 100th on a tapper in front of the plate that catcher Josh Thole threw away for an error.
"Definitely a little relief," McGehee said. "Definitely wasn't how I drew it up."
"It's really rare to do. It really shows the consistency of our offense," Braun said. "That 100th RBI kind of solidifies the fact that we're a run producer."
Narveson matched a season high with nine strikeouts and Lorenzo Cain hit a two-run double after All-Star shortstop Jose Reyes dropped a routine popup, helping the Brewers win for the seventh time in nine games.
The loss ensured the Mets will finish with a second straight losing season. New York (77-82) struck out 13 times in dropping its third in a row to the Brewers.
After finishing 70-92 last year, New York wraps up its second season at Citi Field with a three-game set against Washington.
"We didn't set out to be a .500 club," Mets manager Jerry Manuel said. "We set out to try to win a championship."
The sparse but lively crowd let the Mets know how they felt throughout this comical effort.
With runners at first and third and one out in the second, Alcides Escobar hit a hard, high chopper off third baseman David Wright's glove for a hit that drove in Milwaukee's first run against prospect Dillon Gee (2-2). Jonathan Lucroy followed with an RBI grounder.
In the fifth, Hart hit a two-out pop fly and Reyes barely had to move to settle under it. But the ball hit off his glove and fell to the infield dirt for an error. Cain's double made it 4-1.
"I had it right there," Reyes said. "I don't know how I missed that ball."
The boos for Reyes escalated after he struck out leading off the bottom half of the fifth.
The Mets cut it to 4-2 in the seventh on Thole's RBI double. But in the ninth, it all fell apart for New York.
Reliever Sean Green walked pinch-hitter Chris Dickerson with one out and then hit Rickie Weeks with a pitch. After Braun singled in a run, McGehee, batting in the No. 3 hole for a shot at his 100th RBI, hit a tapper in front of home plate.
Thole, the catcher, threw it wide of first for an error allowing Weeks to scamper home. Ike Davis made a poor throw home to allow the runners to advance to second and third. McGehee could be seen on the replay smiling and mouthing to his dugout: "Was it a hit?"
Hart and Cain had RBI hits and Manny Acosta walked Lucroy with the bases loaded for another run. Three of the five runs were earned.
"As soon as I hit it I was like, 'Yeah!" Hart said.
Narveson got a sliding catch from Braun in left field to start the seventh and needed little else from his fielders in making a strong bid for a spot in the rotation next season. Prince Fielder did make a lunging grab behind first base of a stray piece of paper caught in the wind that was gusting up to 39 mph, getting a nice cheer from the sparse but lively crowd.
Narveson gave up four hits. He won for the second time in six September starts but came in with a 3.52 ERA in the month.
"I hope he realized he pitched well," Brewers manager Ken Macha said.
Gee labored through six innings in his fifth big league start. He yielded seven hits, three walks and four runs -- two earned.
Brewers owner Mark Attanasio thanked his players in a clubhouse address before the game. Attanasio said he'll meet with general manager Doug Melvin on Friday in New York to discuss planning for next season. ... Brewers manager Ken Macha said LHP Randy Wolf will start Sunday. ... Mets OF Nick Evans had an MRI exam on his left, non-throwing shoulder, and results were negative.