Draft and minor leagues The
New York Mets selected Alonso in the second round of the
2016 Major League Baseball draft with the 64th overall pick. He signed with the Mets for a $909,200
signing bonus, and made his professional debut in 2016 with the
Brooklyn Cyclones of the
Class A Short Season New York–Penn League. He posted a .322 batting average with 5 home runs and 21 RBIs in 30 games and was chosen to participate in the league's
All-Star Game. 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, where he batted .311 with 2 home runs and 5 RBIs in 11 games. in 2018
MLB.com ranked Alonso as New York's fourth-best prospect going into the 2018 season. He began the season in Binghamton, and received a midseason promotion to the
Las Vegas 51s of the
Class AAA Pacific Coast League. He represented the Mets in the 2018
All-Star Futures Game. In 132 games between Binghamton and Las Vegas, he
slashed .285/.395/.579 with 36 home runs and 119 RBIs. He won the
Joe Bauman Home Run Award. 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.
New York Mets (2019–2025) 2019 Alonso made the Mets
Opening Day roster in 2019 as the starting first baseman. 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 8th inning. On April 1, Alonso hit his first major league home run, off of
Drew Steckenrider of the
Miami Marlins leading the Mets to a 7–3 victory. On April 9, he had his first career multi-home run game against the
Minnesota Twins in a Mets 14–8 loss. 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. In April, he batted .292 with 9 home runs, 8 doubles, one triple and 26 RBIs in his first games in the big leagues. With his performance, Alonso won the
National League Rookie of the Month Award for April. His 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). 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. On June 30, Alonso was selected as a reserve to the
2019 MLB All-Star Game. He also won the NL Rookie of the Month Award for the month of June. 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 July 8, Alonso won the
2019 Home Run Derby, defeating
Toronto Blue Jays first 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. On August 15, Alonso hit his 39th home run, tying
Cody Bellinger for the most home runs by a rookie in
National League history. Alonso finished the game with a career-high 5 hits and 6 RBIs. He broke Bellinger's record on August 18 with his 40th home run in an 11–5 win against the
Kansas City Royals. 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. 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. He hit his 53rd the next day off of
Atlanta Braves pitcher
Mike Foltynewicz to break Judge's record. In his rookie season, Alonso played in 161 games, batting .260/.358/.583 with 53 home runs and 120 RBI. His 53 homers led all of MLB for the
2019 season. After the season on November 11, Alonso was awarded the
National League Rookie of the Year, receiving 29 of 30 first-place votes. On December 10, he was named to the first team of the inaugural
All-MLB Team.
2020 On September 3, 2020, Alonso hit his first career
walk-off home run in a 9–7 victory against the
New York Yankees. His sophomore season was a step back from his breakout rookie year; Danny Abriano of
Yahoo Sports called it "a season to forget." However, he had the highest maximum
exit velocity of all major league hitters, at . In the
2020 season shortened by
COVID-19, Alonso played in 57 games and batted .231/.326/.490 with 16 home runs and 35 RBIs. 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.
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. He became the third player ever to win back-to-back Derbies, as well as the fourth player to win two Derbies (after
Ken Griffey Jr.,
Prince Fielder, and
Yoenis Céspedes).
Newsday Mets beat writer Tim Healey noted Alonso has earned $2 million from winning the Derby twice while making $1.47 million from 2019–2021 in base salary while playing for the Mets. 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. 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 one-year, $7.4 million contract with the Mets avoiding salary arbitration. He hit his first career
grand slam on April 9 against the
Washington Nationals off of pitcher
Joan Adon in the 5th inning, leading the Mets to a 5–0 victory. in 2022 On July 10, Alonso was named to the
2022 MLB All-Star Game, the second selection of his career. He participated in the
Home Run Derby, making it to the semifinals, but lost to
Julio Rodríguez. In a September 25 game against the
Oakland Athletics, Alonso drove in 5 runs, setting a new Mets' single-season RBI record with 125. He broke the record on a two-run home run in the 4th inning off of pitcher
Norge Ruiz. The Mets previous RBI record was held by
Mike Piazza (1999) and
David Wright (2008). 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. On October 8, during Game 2 of the
NL Wild Card Series, Alonso hit the first postseason home run of his career, a tiebreaking solo shot in the 5th inning off of
San Diego Padres reliever
Nick Martinez, leading the Mets to a 7–3 victory. Alonso played in 160 games for the Mets in 2022, batting .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.
2023 On January 13, 2023, Alonso and the Mets agreed to a one-year, $14.5 million contract during the arbitration period. On April 7, during a game against the
Miami Marlins, Alonso hit his 150th career home run, passing
Carlos Beltrán for 6th place on the 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). 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. He was expected to miss 3–4 weeks due to the injury, but recovered in just eleven days and was activated from the IL on June 18. On July 2, Alonso was named to the
2023 MLB All-Star Game for the third selection of his career. He and
Keith Hernandez were the only two Mets first basemen to be named to three
All-Star teams. On September 3, during a game against the
Seattle Mariners, Alonso homered twice and drove in 4 runs, reaching 40-plus home runs and 100 RBIs for the second consecutive season. His 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 (4 seasons),
Eddie Mathews,
Ryan Howard, and
Albert Pujols (each with 3 seasons). On September 27, Alonso hit his 46th home run of the season in an 11–2 win over the
Miami Marlins. His 192nd career homer made him tie
Howard Johnson for 4th place on the Mets all-time home run list. Alonso played in 154 games for the Mets in 2023, batting .217/.318/.504 with 46 home runs and 118 RBI. 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. He won
NL Player of the Week honors on April 15. On April 27 against the
St. Louis Cardinals, Alonso hit his 200th career home run off of pitcher
Sonny Gray, becoming the 4th-fastest player in MLB history to hit 200 home runs and the 4th Met to hit 200 career home runs. On May 14, 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. On July 7, Alonso was named to the
2024 MLB All-Star Game for the fourth selection of his career. He passed
Keith Hernandez for the most appearances by a Mets first baseman. Two days later, he would hit his 221st career homer, passing Piazza. In 2024, Alonso played in all 162 games for the Mets, making him the second player in Mets history to appear in every game in a single-season, joining
Félix Millán (1975). He finished the season batting .240/.329/.459 with 34 home runs and 88 RBI. On October 3, in Game 3 the
2024 National League Wild Card Series against the
Milwaukee Brewers, with the Mets down 2–0 in the top of the 9th inning, Alonso hit a go-ahead, three-run home run to give the Mets a 3–2 lead. They would then go on to win 4–2 and eliminate the Brewers from the playoffs. 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. In the
2024 MLB postseason, Alonso played in 13 games for the Mets, batting .273/.431/.568 with 4 home runs and 10 RBI. After the 2024 season, Alonso's contract with the Mets expired and became a
free agent for the first time in his career. The Mets offered him a
qualifying offer, which he declined.
2025 On February 12, 2025, after three months of back-and-forth negotiations with his agent
Scott Boras, Alonso and the Mets agreed to a two-year, $54 million contract including an opt-out after the 2025 season. On April 29, Alonso hit the 614th RBI of his career, a solo home run in an 8–3 victory over the
Arizona Diamondbacks, tying
Ed Kranepool for fifth place in franchise history. He passed Kranepool with a two-run homer against the
St. Louis Cardinals three days later. By the end of April, he was slashing .342/.475/.649 with a 1.124 OPS, 7 home runs, and 28 RBI. With his performance, Alonso was named the
National League Player of the Month for April for the first time in his career. On June 1 against the
Colorado Rockies, Alonso hit a three-run home run off of pitcher
Carson Palmquist in the 4th inning, passing
Howard Johnson for 4th place on the Mets all-time RBI list with 632. On June 8 against the Rockies at
Coors Field, Alonso hit a two-run homer off of pitcher
Tyler Kinley in the 8th inning, passing former captain
David Wright for 2nd place on the Mets all-time home run list with 243. Between June 2 and 8, he went 12-for-30 (.400) with two doubles, 5 home runs, and 15 RBI in 7 games. Alonso was named the
NL Player of the Week for that span. On July 6, Alonso was named to the
2025 MLB All-Star Game, the fifth selection of his career. During the sixth inning, Alonso gave the
National League a 5–0 lead after hitting a three-run home run off of
Kansas City Royals pitcher
Kris Bubic, becoming the third Met to homer in the
All-Star Game, joining
Lee Mazzilli (1979) and
David Wright (2006). On August 2 against the
San Francisco Giants, Alonso hit a three-run home run off of pitcher
Kai-Wei Teng in the 1st inning, for his 250th career home run. He became the second player in franchise history to accomplish the feat along with
Darryl Strawberry, who reached that mark on September 18, 1990. Facing the
Atlanta Braves at
Citi Field on August 12, Alonso hit his 253rd career home run to break Strawberry's record, a two-run shot off of
Spencer Strider in the 3rd inning. He also hit his 254th homer off of pitcher
Austin Cox in the 6th inning, for a total of 3 RBI on the day. Alonso played in all 162 games for the Mets in 2025, slashing .272/.347/.524 with 38 home runs and 126 RBI. He led the National League with a career-high 41 doubles. On September 28, after the final game of the season, Alonso stated that he would opt out of his contract and elect free agency. On November 3, 2025, Alonso opted out of his 2026 contract and became a free agent.
Baltimore Orioles (2026–present) On December 11, 2025, Alonso signed a five-year, $155 million contract with the
Baltimore Orioles. ==International career==