In the first Citi Field game in front of fans in over 18 months, it was the Mets by an elbow.

Michael Conforto got called out on strikes in the ninth inning and then he didn’t. Plate umpire Ron Kulpa went from signaling a punch out to ruling that Anthony Bass’ slider clipped Conforto’s right elbow guard, sending the Mets into a late-afternoon celebration Thursday that seemed unlikely when the inning started.

Conforto was leaning into the pitch as he got clipped, leaving the Marlins to cry foul as the winning run in the Mets’ 3-2 victory was forced home. And Kulpa later admitted to a pool reporter that he blew it.

“The guy was hit by the pitch in the strike zone,” Kulpa said. “I should have called him out.”

Conforto was happy with the outcome, but would have preferred a different manner of execution.

“Obviously not the way I wanted to win the ballgame,” he said. “I wanted to go up there and put the ball in play, drive the ball somewhere.

“I knew there was going to be some controversy. Our first-base coach was yelling at me to get down there and touch the base, and ‘Let’s get out of here.’ A win is a win, it’s over, but I would like to use the bat next time for sure.”

Marlins manager Don Mattingly argued the call, but was told the umpires could only review whether Conforto was hit by the pitch, not whether he leaned into it. The replay confirmed the hit by pitch.

“We see a strike,” Mattingly said. “That’s what you end up seeing, a ball in the strike zone. I felt like he turned into it, tried to get hit. The one where he got hit early in the game, we thought he also made a clear move to get hit, try to get hit.”

Mattingly called it “frustrating” that a ball in the strike zone wasn’t called as such.

“[Conforto] got hit by a strike,” Mattingly said. “[Kulpa] went to call it and then he said it hit him.”

Conforto denied he was trying to get hit.

“I was really in two-strike battle mode,” he said. “With two strikes I just went into battle mode and I tend to lean over the plate when I get into battle mode.”

On his 29th birthday, Jeff McNeil blasted a game-tying homer against Bass leading off the ninth to help rescue the Mets, who had only four hits to the point. McNeil had been 0-for-10 to start the season.

After the homer, the Mets continued the rally with a Luis Guillorme one-out single and Brandon Nimmo double. Francisco Lindor was intentionally walked to load the bases for Conforto, who heard a smattering of boos in his previous at-bat when he hit into an inning-ending double play. But the pandemic-restricted sellout crowd of 8,011 still left happy.

“When Jeff hit that homer the place just exploded,” Conforto said. “I know we’re at limited capacity, but that was fun, to see Jeff waiting to get that first hit and emotional after tying that game up. That was fun to see and the roar from the crowd. We have been missing that, so that was great.”

In his Mets debut, Taijuan Walker lasted six innings and allowed two earned runs on four hits and two walks with four strikeouts. Walker had thrown a simulated game last weekend in Washington to keep sharp, after the opening series was postponed. He was removed after 87 pitches Thursday. It was only the second career Citi Field start for Walker, who had last pitched in the ballpark with the Diamondbacks in 2017.

Jesus Aguilar slapped an RBI single in the sixth for the Marlins’ second run of the inning, putting the Mets in a 2-1 hole. Corey Dickerson doubled to bring in the first run, after Jon Berti singled leading off the inning.

Walker held the Mets hitless into the fifth. Brian Anderson singled with one out to end the early no-hit watch and Jazz Chisholm Jr.’s walk gave the Marlins a scoring threat before Walker retired Chad Wallach to escape the inning.

The Mets loaded the bases in the fifth, and Dominic Smith’s drive to center had potential to clear the fence. But Starling Marte raced to the 408-foot sign and caught the ball, limiting the damage to a sacrifice fly. The ensuing batter, Jonathan Villar, struck out against Ross Detwiler to keep the Mets’ lead at 1-0.

Miguel Castro and Trevor May were both used for the third time in four days. Each pitched a shutout inning before Edwin Diaz’s season debut. Diaz, who earned the win, pitched a scoreless ninth with one walk and one strikeout.

Then came the Mets rally and celebration.

“It was louder than I expected,” McNeil said. “When they announced it was going to be 20-25 percent [capacity] I didn’t think it would be that loud or anything. But you know Mets fans, they are loud, they are fun, it was more than I expected for sure.”