Alonso's bases-loaded home run came in the fifth inning of the 8-4 win Thursday in Port St. Lucie, Fla.
"Big shoutout: Happy birthday, mom," Alonso told reporters. "I'm glad I could get a birthday bomb for you. Thank you guys for coming. It was really nice to have my parents at the game."
The Mets trailed 4-1 entering the bottom of the fifth frame. Khalil Lee flew out to right field to start the half-inning. Drew Ferguson then singled for the Mets.
Nationals pitcher Cole Henry then issued a walk to Brandon Nimmo and hit Jeff McNeil with a pitch to load the bases.
Dominic Smith struck out in the next at-bat, which prompted Alonso to step into the batter's box. The Mets first baseman proceeded to smash a Henry offering to right field for a 428-foot homer. The blast traveled 113 mph as it left the park, according to Statcast.
"I'm really happy for being able to drive the ball to the big part of the field in center field or dead right field," Alonso said. "I pride myself on being able to take the ball to the big part of the yard. If I hit it hard, it's going to go a long way."
Jake Hager belted a two-run homer for the Mets in the bottom of the seventh inning. The Mets scored their final run on a bases-loaded walk in the eighth frame.
Nimmo also had a solo home run in the bottom of the first inning to give the Mets an initial lead. Ryan Zimmerman hit a three-run homer for the Nationals in the top of the fifth inning.
The Mets face the Houston Astros in another spring training game at 6:05 p.m. EST Friday at the Ballpark of the Palm Beaches in West Palm Beach, Fla.