MLB.com ranked Alonso as New York's fourth best prospect going into the 2018 season.[18] He began the 2018 season in Binghamton,[19] and received a midseason promotion to the Las Vegas 51s of the Class AAAPacific Coast League. He represented the Mets in the 2018 All-Star Futures Game.[20] In 132 games between Binghamton and Las Vegas, he slashed .285/.395/.579 with 36 home runs and 119 RBIs.[21] He won the Joe Bauman Home Run Award.[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.[23]
2019
Alonso made the Mets Opening Day roster in 2019 as the starting first baseman.[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.[25] First publicized on March 30, third-base coach Gary DiSarcina gave Alonso the endearing nickname "Polar Bear" for his power.[26] He hit his first major league home run on April 1 off of Drew Steckenrider of the Miami Marlins.[27] On April 9, Alonso had his first career multi-home run game against the Minnesota Twins.[28]
Alonso became the first player in MLB history since 1900 with 11 extra-base hits in his first 10 career games. No other player ever had more than nine.[29] In April, Alonso batted .292 with nine home runs, eight doubles, one triple and 26 RBIs in his first games in the big leagues. He won the National League Rookie of the Month Award for April.[30] 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).[31]
On June 22, Alonso hit his 26th home run, breaking the National League record for most home runs by a rookie before the All-Star break, passing Cody Bellinger. The next day, with his 27th home run, he broke the Mets rookie home run record, previously set by Darryl Strawberry in 1983.[32][33] Alonso was selected as a reserve to the 2019 All-Star Game.[34] He also won the National League Rookie of the Month Award for the month of June.[35]
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).[36] On July 8, Alonso won the 2019 MLB Home Run Derby, defeating Toronto Blue Jays third baseman Vladimir Guerrero Jr. 23–22 in the final round, becoming only the second rookie, after Aaron Judge, to win the Derby outright. 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.[37]
On August 15, Alonso hit his 39th home run, tying Bellinger for the most home runs by a rookie in National League history. Alonso finished the game with a career-high five hits and six RBIs.[38] He broke Bellinger's record on August 18 with his 40th home run in an 11–5 win against the Kansas City Royals.[39] On August 27, Alonso hit his 42nd home run to become the Mets all-time single-season home run leader, surpassing Carlos Beltrán and Todd Hundley. He is the first rookie to set his franchise's single-season home run record since Johnny Rizzo did it for the Pittsburgh Pirates in 1938.[40]
On September 27, Alonso hit his 52nd home run of the season, tying Aaron Judge for the most home runs by a rookie in MLB history.[41] He hit his 53rd the next day to break Judge's record.[42] His 53 home runs led all of Major League Baseball for the 2019 season. On November 11, Alonso was awarded the National League Rookie of the Year, receiving 29 of 30 first-place votes.[43][44] On December 10, Alonso was named to the first team of the inaugural All-MLB Team.[45]
2020
In the 2020 season shortened by COVID-19, Alonso played in 57 of the 60 games and batted .231/.326/.490 with 16 home runs and 35 RBIs.[46]
Alonso with the Mets in 2020
On September 3, 2020, Alonso hit his first career walk-off home run.[47] Alonso's sophomore season was a step back from his breakout rookie year; Danny Abriano of Yahoo Sports called it "a season to forget."[48] However, he had the highest maximum exit velocity of all major league hitters, at 118.4 miles per hour (190.5 km/h).[49]
Alonso's sophomore slump coupled with a second consecutive breakout season from Dominic Smith led some talent evaluators to favor the latter as the team's starting first baseman heading into the 2021 season.[50]
2021
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.[51] He became the third player ever to win back-to-back Home Run Derbys, as well as the fourth player to win two Home Run Derbys (after Ken Griffey Jr., Prince Fielder, and Yoenis Céspedes). Newsday Mets beat writer Tim Healey noted Alonso has earned $2 million from winning the Home Run Derby twice while making $1.47 million from 2019–2021 in base salary while playing for the Mets.[52]
On September 7, Alonso hit his 100th career home run off of Miami Marlins pitcher Edward Cabrera. He accomplished the feat in 347 games, becoming the second-fastest player to reach 100 career home runs behind Ryan Howard, who did it in 325 games.[53]
Alonso played in 152 games for the Mets in 2021, batting .262/.344/.519 with 37 home runs and 94 RBIs.
2022
On March 22, 2022, Alonso signed a $7.4 million contract with the Mets, avoiding salary arbitration.[54] He hit his first career grand slam on April 9 during a game against the Washington Nationals off of pitcher Joan Adon.[55]
Alonso rounding the bases after hitting his first career grand slam in 2022
On July 10, Alonso was named to the 2022 MLB All-Star Game, the second selection of his career,[56] He participated in the Home Run Derby, making it to the semifinals.[57] 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] He broke the record on a 451-foot (137 m) two-run home run in the fourth inning off of pitcher Norge Ruiz. The Mets previous RBI record was held by Mike Piazza (1999) and David Wright (2008).[59]
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.[60]
Alonso ended the season with 131 RBIs, breaking the franchise record and tying Aaron Judge for the most in the major leagues.
2023
Alonso and the Mets agreed to a $14.5 million salary for the 2023 season during the arbitration period.[61] On April 7, during a game against the Miami Marlins, Alonso hit his 150th career home run, passing Carlos Beltrán for sixth place on Mets all-time home run list. He also became the second fastest player to 150 home runs (in 538 games) since Ryan Howard (495 games).[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.[64]
On September 3, during a game against the Seattle Mariners, Alonso homered twice and drove in four runs, reaching 40-plus home runs and 100 RBIs for the second consecutive season. Alonso's two-run shot made him the fifth player in Major League history with at least three 40-home run seasons in his first five seasons of their career, joining Ralph Kiner (four seasons), Eddie Mathews, Ryan Howard, and Albert Pujols (each with 3 seasons).[65]
2024
The Mets and Alonso agreed on a one-year, $20.5 million contract to avoid salary arbitration on January 11, 2024.[66] On April 15, he earned NL Player of the Week honors.[67] On April 27, during a game against the St. Louis Cardinals, Alonso hit his 200th career home run off of pitcher Sonny Gray. Alonso became the fourth fastest player in MLB history to hit 200 home runs and the fourth Met to hit 200 career home runs.[68][69]
Alonso married Haley Walsh on November 12, 2021, at Sacred Heart Catholic Church in downtown Tampa. She is originally from Boston; they met in the summer of 2015 while he was playing in the Cape Cod collegiate summer league. They carried on a long-distance relationship while he continued at the University of Florida and she attended Michigan State University. They were engaged in November 2018.[73][74]
In 2020, the couple formed The Alonso Foundation, a charitable organization that supports youth, veteran and animal causes.[75]
On March 13, 2022, Alonso was involved in a car accident in Tampa, Florida, which he described as a "really close experience to death." A driver ran a red light and struck Alonso's vehicle, which flipped over three times. Neither Alonso nor anyone else involved in the incident was injured.[76]