A good start can do a lot.

It can provide cover for holes in the bullpen.

It can serve as camouflage for an offense that is not hitting on all cylinders.

The Mets hadn’t experienced a strong start since David Peterson was performing like he did in the first 2 ¹/₂ months of the season, when he seemed a strong candidate for an All-Star Game selection.

On Thursday, that version of Peterson returned, highlighting that the major contrast between the Mets when they were succeeding and their recent struggles was a rotation that had collapsed in recent weeks.

Following two poor showings, Peterson was once more outstanding and durable, leading the Mets to a series-clinching 3-2 victory over the Brewers in front of a lively, full house of 42,241 at Citi Field.

The Mets (50-38) finally won a series — their first since sweeping the Nationals way back from June 10-12 — and pulled out just a fifth game in their past 19, building momentum for the Queens portion of the Subway Series that begins Friday.

Among the many issues facing the Mets, the largest has been a rotation that has been battered both by injuries and opposing batters.

Before Peterson battled for 6 ²/₃ innings Thursday, manager Carlos Mendoza had not seen his starting pitcher survive six innings since … Peterson went seven innings in Atlanta on June 17.

From June 18 through Wednesday, the Mets’ rotation posted a 5.96 ERA that was the third-worst in MLB while tallying 54 ¹/₃ innings, the third-least.

Included in that stretch were two Peterson starts, one in Philadelphia and one in Pittsburgh, in which he allowed 10 runs in 8 ²/₃ innings.

“On Petey I’m not concerned. This is a very good major league starter,” David Stearns said of Peterson, whose slump is over after allowing two runs (one earned) while scattering five hits, walking three, striking out four and inducing ground balls at will against a solid Milwaukee offense.

The big bats in the Mets’ offense did enough to ensure his work mattered.

Brandon Nimmo lasered a homer for a second straight day to seize a lead in the second inning, an edge that would disappear on an unearned run in the fourth.

Rhys Hoskins lined directly to second baseman Brett Baty, who went to his knees and watched the ball graze off his glove for an error. With two outs and runners on the corners, Caleb Durbin hit a slow roller that just got by Peterson, and Mark Vientos’ throw was too late on an infield single that tied the game.

But the Mets went ahead to stay with a two-run sixth, when the top of the order — new-look leadoff hitter Starling Marte, Francisco Lindor, Juan Soto and Pete Alonso — stitched together four straight hits, Soto’s single and Alonso’s double off the wall in left-center driving in the runs.

Peterson allowed a second run in the seventh, when Andruw Monasterio turned on a four-seamer that was not far enough inside for a home run.

But Ryne Stanek (1 ⅓ dominant innings with three strikeouts) and Edwin Díaz (18th save, with some help from a perfect throw to second from Luis Torrens and a perfect tag from Lindor that caught Christian Yelich stealing) ensured the Brewers got no closer.