Alonso's high drive to left field off Josiah Gray made it 4-3 and kicked off a four-run inning. He has 32 homers and leads the NL with 106 RBIs, 18 short of the team record shared by Mike Piazza in 1999 and David Wright in 2006.
''Anybody analytically that tells you it's not real, it's real - guys have a different level when there's RBIs out there and people on base and teammates, more importantly,'' Mets manager Buck Showalter said.
''You don't drive in 106 or whatever RBIs he's got now without having that ability to step up your concentration level in times of need.''
The NL East-leading Mets have 30 games left for Alonso to make a run at the mark.
''Him driving in 100, it's special,'' said Mets shortstop Francisco Lindor, who has 86 RBIs. ''It's a tough task to do.''
Eduardo Escobar hit his first homer since July 26 and drove in three runs for New York. Escobar, who had been losing playing time to Luis Guillorme and Brett Baty before they hit the injured list, is batting .217, 36 points below his career average.
''A game like today really helps a lot with a player's confidence,'' Escobar said through a translator. ''This year, I obviously haven't had the result that I've wanted.''
Mychal Givens (7-3), who entered with an 8.03 ERA in his first 13 appearances for the Mets, recorded five outs in relief of starter David Peterson. Givens stranded a runner at third in the sixth and worked around a two-out walk in the seventh.
Showalter said three of his top relievers - closer Edwin Diaz and setup men Adam Ottavino and Trevor May - weren't available.
''Givens was really good tonight. That was a key outing for us,'' Showalter said.
Gray (7-9), pitching for the first time since Aug. 20, allowed six runs in five-plus innings for last-place Washington.
It was the first start at Citi Field for Gray, who grew up about 15 miles away in New Rochelle, New York.
''I thought he handled it really well,'' Nationals manager Dave Martinez said. ''He kept us in the ballgame. That sixth inning, (he) comes out and he just made a couple mistakes. Ball started going over the plate, ball started getting up on him and for me that's an indication that he was probably a little bit tired. But other than that, he threw the ball well.''
Jeff McNeil had three hits for the Mets, his 14th multi-hit effort in 33 games dating to July 30. Tomas Nido drove in two runs.
After Alonso's homer in the sixth, Escobar had a sacrifice fly that made it 5-3. Mark Canha was thrown out at second base on the play, but Nido followed with an RBI single and Brandon Nimmo drove him in with a triple.
Victor Robles scored the Nationals' first run by racing home from third on a third-inning throwing error by catcher Nido, who threw wide of the bag after Lane Thomas struck out on a ball that bounced. Thomas was safe at first on the play.
Alex Call had an RBI single in the fifth and Ildemaro Vargas tied the game with a run-scoring double that chased Peterson in the sixth.
Luke Voit finished with three singles for the Nationals.
Peterson was charged with three runs in 5 1/3 innings.
A NATIONAL HOMECOMING
Gray, teammate Josh Palacios and Martinez all had friends and relatives in attendance. Palacios was born and went to high school in Brooklyn while Martinez was born in Brooklyn before moving to Florida as a teenager.
Nationals: DH Nelson Cruz was lifted after he fouled a ball off the bottom of his right knee. Martinez said he's day-to-day. ... LHP MacKenzie Gore (left elbow inflammation) threw a bullpen Friday and could throw a simulated game during Washington's series next week against the St. Louis Cardinals. Martinez said the goal is for Gore - who has been sidelined since the week before he was traded to the Nationals by the San Diego Padres as part of the Juan Soto deal - to make a start before the season ends.
Mets: RHP Tylor Megill (right shoulder strain) threw a perfect inning of relief Thursday for Double-A Binghamton. Showalter said he hoped Megill, who will return as a reliever after making his first 27 big league appearances as a starter, could pitch again Saturday or Sunday.
Nationals: LHP Patrick Corbin (5-17, 6.56 ERA) snapped the Nationals' major league-record streak of 43 straight games without a win by a starting pitcher in his previous start Sunday.
Mets: RHP Max Scherzer (9-4, 2.27) goes against one of his former clubs in his third attempt at earning the 200th win of his career.
More AP MLB: https://apnews.com/hub/mlb and https://twitter.com/AP-Sports
Copyright 2023 STATS LLC and Associated Press. Any commercial use or distribution without the express written consent of STATS LLC and Associated Press is strictly prohibited.