Paul Skenes, phenom and phenomenon, began Monday night as the headliner at Citi Field, an all-everything righthander and burgeoning marketing star who represents about the only glimmer of relevancy for a flailing franchise. When he takes the mound, people care. When he takes the mound, even the Pirates manage to win most of the time.

But not this time.

The Mets added a Skenes start to their list of conquests with a 4-3 walk-off win, another encouraging and well-rounded success following consecutive series victories over the Diamondbacks and Cubs.

Pete Alonso’s sacrifice fly off David Bednar plated Francisco Lindor in the bottom of the ninth. That undid the damage done during the top of the final frame, when misplays by Lindor and Luisangel Acuna allowed Pittsburgh (14-28) to manufacture the tying run.

“Whether it’s Skenes or another ace in the league or anybody else, we like our chances,” Alonso said. “We play really, really well when everyone is having collective at-bats. It doesn’t matter if we’re tied, down or up. If we’re having good collective team at-bats, that’s a really good sign. And we had great at-bats all night. It was just a matter of time.”

For Alonso, it was the eighth walk-off RBI of his career. Wilmer Flores (10) and David Wright (nine) are the only Mets with more.

“He loves those situations,” Brandon Nimmo said. “He wants them.”

In a back-and-forth affair, Nimmo provided the defensive highlight, a leaping catch at the leftfield wall to rob Joey Bart of at least a double and perhaps a home run in the eighth inning.

That preserved what at the moment was a tiny lead for the Mets (27-15). They went ahead in the seventh on a two-run rally capped by Acuna scoring from second on Alonso’s groundball single to the left side of the infield/shallow leftfield.

On a gutsy send by third-base coach Mike Sarbaugh, Acuna barely beat the tag from catcher Henry Davis, who for some reason was set up behind the plate.

“His speed is electric,” Alonso said. “He causes a ton of havoc on the basepaths. He’s a burner and having that skill set is huge for us.”

By his usual standards, Skenes was downright hittable. The Mets just didn’t make much of it, managing one run in six innings. He worked around six hits (three doubles) and three walks, including in the first inning, when Lindor and Juan Soto reached base with no outs but did not score. Skenes struck out six and maxed out at 99.2 mph with his fastball.

The Mets found solace in making him work. This was tied for his third-shortest outing of the year.

“He’s a great arm and I thought we did a great job of grinding away, chipping away,” Alonso said.

Manager Carlos Mendoza said: “We created traffic pretty much every inning. You gotta get the big one. That’s what it’s going to take to beat guys like him. You create traffic and you hope someone clips him.”

The Mets never got the big one, but it didn’t matter. Lefthander David Peterson kept it close — pretty much matching his counterpart — by allowing two runs in six innings (plus one batter). He scattered five hits and three walks and struck out seven.

Pittsburgh’s only run most of the way came via Isiah Kiner-Falefa’s home run in the second. When Jared Triolo opened the seventh by fighting Peterson for a 10-pitch walk, Peterson exited at 100 pitches. Triolo scored against Jose Butto, advancing on a stolen base, a third failed pickoff attempt/balk and a forceout.

“You gotta give credit to Peterson, who pretty much kept us in the game,” Mendoza said. “When you’re facing a guy like that [Skenes], it’s not only the at-bats from the offense, it’s the guy you have on the mound, keeping it close. Petey did that. He went out there and matched him.”

Peterson takes pride in that, this time especially so.

“It’s always fun when you go up against someone’s best,” Peterson said. “It’s fun when you’re just trading blows like that and the offenses are grinding . . . In games like that, it’s a matter of us as a pitching staff holding it together where it is and giving the offense the opportunity to finally break through.”