His mother, Michelle Lynn Alonso (née Morgan), played college softball at Ohio Wesleyan University.

[4] In 2014, he played collegiate summer baseball with the Madison Mallards of the Northwoods League,[7] and he was inducted into their Hall of Fame in 2024.

[15] He began the 2017 season with the St. Lucie Mets of the Class A-Advanced Florida State League, and after batting .286 with 16 home runs and 58 RBIs in 82 games, was promoted to the Binghamton Rumble Ponies of the Class AA Eastern League in August,[16] where he batted .311 with two home runs and five RBIs in 11 games.

[18] He began the 2018 season in Binghamton,[19] and received a midseason promotion to the Las Vegas 51s of the Class AAA Pacific Coast League.

[22] That season, Alonso was the final batter in Cashman Field history, as he hit a walk-off home run in the bottom of the 9th inning during the last baseball game played at the facility.

[24] He was in the team's starting lineup on Opening Day on March 28, and recorded his first major league hit against Washington Nationals pitcher Justin Miller in the eighth inning.

[29] Alonso's nine home runs led all MLB rookies and are tied for the most by a Mets player before May 1 with Neil Walker (2016), John Buck (2013), Carlos Delgado (2006) and Dave Kingman (1976).

[34] On July 7, Alonso became the second player in Mets history to hit the most home runs (30) before the All-Star break, after Dave Kingman (1976).

On the way to the final round, Alonso also beat Carlos Santana and Ronald Acuña Jr., hitting 57 total home runs during the entire Derby.

[49] On July 12, Alonso won his second straight Home Run Derby after defeating Baltimore Orioles first baseman Trey Mancini 23–22 in the final round.

[54] On July 10, Alonso was named to the 2022 MLB All-Star Game, the second selection of his career,[55] He participated in the Home Run Derby, making it to the semifinals.

[56] In a September 25 game against the Oakland Athletics, Alonso drove in five runs, setting a new Mets' single-season RBI record with 125.

[58] On September 27 against the Miami Marlins, Alonso hit his 40th home run of the season, becoming the first Mets player to accomplish the feat twice.

[59] In 2022, Alonso batted .271/.352/.518 with 40 home runs and 131 RBIs, breaking the franchise record and tying Aaron Judge for the most RBI in the major leagues.

[62] On June 7, Alonso was placed on the injured list when he suffered a bone bruise and left wrist sprain after being hit by a pitch.

[63] He was expected to miss three to four weeks due to the injury, but recovered in just eleven days and was activated from the injured list on June 18.

His 192nd career homer made him tie Howard Johnson for 4th place on the Mets all-time home run list.

[69] On April 4, during a game against the Detroit Tigers, Alonso recorded his 500th career RBI on a game-tying home run off of pitcher Alex Faedo, becoming the 10th and fastest player in Mets history to achieve the milestone.

[73][74] On May 13, it was reported online that the Mets made an attempt to extend Alonso the previous season but they were unsuccessful, as he turned down a seven-year, $158 million contract extension from the team.

[80] With that home run, Alonso became the first major league player in history to hit a go-ahead homer while trailing in the ninth inning or later of a winner-take-all postseason game.

[84] On February 12, 2025, after three months of back-and-forth negotiations with his agent Scott Boras, Alonso and the Mets agreed on a two-year, $54 million contract including a player opt-out after the 2025 season.

He made such a big impression during spring training in 2019 that Mets third baseman Todd Frazier, who was known for giving players nicknames, is the one who coined it.

[90][91] Alonso married Haley Walsh on November 12, 2021, at Sacred Heart Catholic Church in downtown Tampa.