Alonso ripped the solo shot in the top of the fifth frame Wednesday at Nationals Park in Washington, D.C.
"It feels pretty good," Alonso told reporters. "I feel really, really fortunate. I'm really happy with how I've been performing.
"This year's not over."
The Nationals jumped out to an early lead on an RBI single from Trea Turner in the first inning. Juan Lagares answered for the Mets in the third frame, hitting his third home run of the season to tie the score at 1-1.
Mets slugger Robinson Cano slugged his 11th homer of the season in the fourth inning, giving New York a 3-1 lead.Jeff McNeil grounded out to lead off the fifth frame, before Alonso walked to the plate. The star rookie settled in against Nationals starter Anibal Sanchez. The right-handed pitcher tossed in an 86.5-mph splitter for the only offering of the exchange.
Alonso sent the pitch over the left field fence, pushing the Mets' lead to three runs. The 408-foot homer had an exit velocity of 106 mph and left the field in 5.7 seconds, according to Statcast.
The Mets scored three more runs in the sixth frame for a 7-1 lead before the Nationals answered with three runs in the bottom of the inning.
McNeil singled in the game's final run with an eighth-inning single.Alonso went 1-for-5 with an RBI and a run scored in the win. He is now hitting .267 with 45 homers and 105 RBIs during his rookie campaign. McNeil was 2-for-5 with two RBIs in the win. Cano went 3-for-3 with two RBIs, two runs scored and a walk for the Mets. Anthony Rendon led the Nationals at the plate, recording a 2-for-5 day with two RBIs in the loss. Sanchez took the loss on the hill, allowing eight hits and seven runs in five innings.
Mets starter Zack Wheeler allowed seven hits and one run in five innings to pick up his 10th win of the season.
The Mets host the Philadelphia Phillies in a three-game series, starting at 7:10 p.m. EDT Friday at Citi Field. The Nationals travel for a road series against the Atlanta Braves, starting at 7:20 p.m. EDT Thursday at SunTrust Park in Atlanta.