Pete Alonso started 2019 not knowing if he’d make the Mets Opening Day roster. He finished it by winning the NL Rookie of the Year Award on Monday.

“Holy expletive,’’ Alonso said on MLB Network after the announcement was made — using the actual word “expletive.”

Alonso, 24, beat out Atlanta right-hander Mike Soroka and San Diego shortstop Fernando Tatis Jr.

The only surprise was that Alonso didn’t join AL Rookie of the Year Yordan Alvarez of the Astros as a unanimous winner.

One first-place vote went to second-place finisher Soroka, with The Athletic’s Andrew Baggarly pointing to Soroka’s ability to limit home runs in the year of the homer.

Alonso said he was unbothered by missing out by one vote.

“I wasn’t expecting anything like that,’’ Alonso said on a conference call. “To just win the award, it doesn’t matter if it’s unanimous or not. … They’re awesome players and absolutely they should have some votes.”

Alonso is the first Met to win the award since Jacob deGrom in 2014 and the sixth Met to win it, joining Tom Seaver (1967), Jon Matlack (1972), Darryl Strawberry (1983), Dwight Gooden (1984) and DeGrom. DeGrom has a chance to win a second consecutive NL Cy Young Award on Wednesday.

The selection was expected after Alonso broke Judge’s MLB rookie record for homers with 53 and led all rookies with games played, RBIs, runs scored and walks.

Only Tatis finished with a higher OPS among rookies, and Tatis’ season was cut short by a stress reaction in his back in August.

The award was the culmination of a breakout season for Alonso, who spent spring training trying to make the team, since the Mets could have saved a year of control by keeping him in the minors to start the season and there were questions about his defense.

Instead, new general manager Brodie Van Wagenen rewarded Alonso for his production at the plate after he hit a combined 36 homers in the minors, which didn’t even get him a September call-up in 2018.

Alonso credited Van Wagenen for showing confidence in him after the former agent became the Mets’ general manager last year.

“I’m forever grateful for that, for him believing in me after a frustrating 2018 in the minors,’’ Alonso said on SNY. “Him keeping his word. I just wanted to make the most of it.”

Alonso went on to win NL Rookie of the Month in March/April and set a franchise record for homers, shattering the mark of 41 shared by new manager Carlos Beltran and Todd Hundley. Alonso also won the Home Run Derby in Cleveland.

“Opening Day, I was hitting second against Max Scherzer and that’s pretty much all the confidence I needed,’’ Alonso said of his attitude in his first year in the majors. “If I’m there, I might as well do the best I can to do my best to perform and not go back to the bushes.”

That was never an issue.

And now Alonso has bigger things in mind.

“I know next year, success isn’t just given,’’ Alonso said. “I can’t just rely on my reputation from this year. I’m gonna work just as hard, if not harder. I love to compete and I love to win. Winning championships, that’s gonna be the goal next year.’’

Van Wagenen and team COO Jeff Wilpon praised Alonso in statements released by the team, with Van Wagenen saying: “Pete was a joy to watch all season long for our passionate fans as well as all of us in the organization. We’re so proud to see his on-field results match his tireless work ethic.”

Alonso will receive the award at the New York Baseball Writers’ Dinner on Jan. 25 in Manhattan.