The game was suspended on Friday night after inclement weather hit with one out in the bottom of the fourth inning.
New York was up 4-3 when play resumed Saturday, and the Mets added to their lead in the sixth after Alonso doubled, took third on a wild pitch and scored on Brett Baty's RBI single. Prior to hitting the double, Alonso had gone 44 plate appearances without an extra-base hit
Alonso also singled in the eighth inning.
Boston pulled within a run when Triston Casas led off the seventh with a triple and scored on Rob Refsnyder's sacrifice fly. That capped the scoring, however, and the loss stretched Boston's losing streak to three games.
Grant Hartwig (3-1) tossed two innings of scoreless relief to earn the win. David Robertson picked up his 14th save by pitching a scoreless ninth.
The Red Sox led 3-0 Friday night until Brandon Nimmo and Daniel Vogelbach each belted a two-run homer for the Mets. Nimmo's blast, his 15th of the season, sliced Boston's lead to 3-2 in the third inning, and Vogelbach's sixth home run of the season put the Mets in front in the fourth
Both home runs came against Red Sox starting pitcher Kutter Crawford (4-5), who surrendered four runs on as many hits in four innings. He struck out two and didn't walk a batter
New York starter Kodai Senga gave up three runs on three hits, struck out three and walked one in 3 1/3 innings.
The Red Sox took a 1-0 lead when Jarren Duran scored from third on Justin Turner's ground out in the first, and led 3-0 after Yu Chang's two-run double drove in Connor Wong and Casas in the third
Mets second baseman Luis Guillorme left the game in the sixth inning Saturday after he injured his foot attempting to field a ball hit by Rafael Devers
The teams will play the second contest of the three-game series Saturday night.
--Field Level Media