NEW YORK — Mere months ago, a night like this wouldn’t have seemed possible for the Mets.

Jonah Tong wasn’t even invited to big- league spring training.

The first time he met manager Carlos Mendoza, Tong was serving as the ballboy in a Triple-A game during camp, fulfilling a right of passage as one of the newbies.

Tong began the regular season with Double-A Binghamton, where, to that point, he had made only two starts.

He was light years away from the majors.

But fast forward to Friday night, and there Tong was on the Citi Field mound, making his much-hyped MLB debut against the Miami Marlins.

And Tong demonstrated much of the promise that’s turned him into one of the Mets’ most prized prospects.

Tong allowed four runs (one earned) over five innings in the Mets’ 19-9 victory, striking out six, on a night the Mets set a franchise record for runs in a home game. Tong gave up six hits but did not issue a walk over 97 pitches and picked up the win.

The 22-year-old right-hander began his debut with four scoreless innings, then allowed an RBI single to Eric Wagaman to start the fifth. The Mets’ defense let Tong down from there, with back-to-back errors by Francisco Lindor and Pete Alonso extending the inning and leading to the three unearned runs.

Still, Tong ended his inaugural outing on a high note, freezing Liam Hicks with a 3-2 fastball on his final pitch of the night.

He exited to an ovation from a sellout crowd of 42,112 that had cheered loudly for Tong all night, including as he walked to the bullpen for his pregame warm-up and when he struck out Joey Wiemer in the third inning for his first career K.

Tong deployed a four-pitch arsenal, but he relied the most on his mid-90s fastball, throwing it 61% of the time. He struck four batters out looking with that fastball in a nod to how deceptive the pitch can be.

Behind an unusual, over-the-top delivery that strongly resembles that of Tim Lincecum, Tong led the minor leagues in ERA (1.43) and strikeouts (179) over 22 starts at two different levels.

Only two of those starts came at Triple-A Syracuse — where he was promoted earlier this month — but Tong’s overwhelming dominance left the Mets little choice but to bring him up to the majors.

“If you had asked me at the beginning of the year if I thought that this was going to happen, I would tell you exactly what I’m going to tell you now: I’m going to be where my feet are,” Tong said Thursday upon arriving at Citi Field. “I think everything else is going to take care of itself.”

Helping to ease Tong in was a Mets offense that showered him with immediate run support. Before Marlins starter Eury Perez even recorded an out, the Mets led 5-0 behind a two-run home run from Juan Soto and a three-run blast from Brandon Nimmo.

Alonso followed with a two-run home run against reliever Tyler Zuber to kick off a seven-run second inning, after which the Mets led 12-0.

The Mets’ previous record for runs in a home game was 17. On Friday, they finished with 18 hits, including six home runs.

Nimmo added a sixth-inning solo homer off of the right-field foul pole against reliever Valente Bellozo. Nimmo went 3 for 4 with a walk and four RBIs. He now has 22 home runs — two shy of his career high.

The Mets then piled on with a six-run rally in the eighth inning against Javier Sanoja, a position player pressed into mop-up pitching duty. Mark Vientos clubbed a solo home run off of Sanoja, and catcher Luis Torrens added a three-run blast.

Torrens then took the mound for the ninth, but he surrendered four runs — including two homers — in 1/3 of an inning before reliever Ryne Stanek got the final two outs.

The win kept the Mets (73-62) five games behind the NL East-leading Philadelphia Phillies, who also won Friday. The Mets now have a five-game lead for the third and final NL wild-card spot over the Cincinnati Reds, who lost Friday.

Tong is the second Mets mega-prospect promoted this month in the heart of the playoff push, following 24-year-old right-hander Nolan McLean, who is 3-0 with a sterling 0.89 ERA through three starts.

But while McLean is locked into the rotation moving forward, the Mets have not committed to Tong beyond Friday’s start.

“I think as you roll into the last month of the season, you want to have the best roster you possibly can,” president of baseball operations David Stearns said this week. “We think Jonah has a chance to be among that.”

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