Cincinnati Reds in In 2008, Alonso was drafted for the second time with the seventh overall pick by the
Cincinnati Reds. He signed with the Reds for $4.55 million. Alonso made his
Minor League debut on August 26, 2008, for the
High–A Sarasota Reds against the
Clearwater Threshers. He went 1–for–3 with a double and a walk. In just six games for Sarasota, he went 6–for–19 with two RBI. Alonso entered the 2009 season ranked as the number one overall prospect in the Reds organization by
Baseball America. He also played for the
Peoria Saguaros of the
Arizona Fall League in 2009, .267 in 23 games. In May, Alonso was promoted to the Triple–A
Louisville Bats where he played 82 of his 101 games at first base, hitting .296 with 12 home runs. Alonso was called up by the Reds on September 1 after the rosters expanded. He made his major league debut that day, pinch hitting for
Brandon Phillips. On September 4, in his second at bat as a pinch hitter, Alonso doubled off
St. Louis Cardinals pitcher
Mike MacDougal for his first major league hit, and scored his first major league run that inning. Alonso began the 2011 season at Triple–A, playing the majority of his games in left field and batting .296. On July 26, 2011, Alonso was recalled to the Reds after they traded
Jonny Gomes. He served mostly as a pinch hitter, picking up 98 plate appearances over 47 games and batting .330 with a .943
OPS. Alonso made 14 starts in left field and one each at first and third base. Alonso was projected as a left fielder for the Reds in 2012, although he struggled defensively at the position during 2011.
San Diego Padres Alonso,
Edinson Vólquez,
Yasmani Grandal, and
Brad Boxberger were traded to the
San Diego Padres in exchange for
Mat Latos on December 17, 2011. He was projected to start at first base ahead of
Anthony Rizzo, who was eventually traded. Alonso came into 2012 still classified as a rookie with only 117 career at–bats. Alonso played 155 games for the Padres in 2012, including 144 starts at first base, and posted a batting line of .273/.348/.393 with nine home runs. He set a Padres franchise record with 39 doubles as a rookie, breaking the old rookie mark of 33 by
Benito Santiago in 1987. Alonso also had 10 multi–double games, the most in the major leagues since 2006. Alonso committed 12 errors at first base, tying for the league lead. He finished 6th in the NL Rookie of the Year voting. Alonso was the starting first baseman to begin 2013. On April 10, Alonso started at first base against the Dodgers. In the 9th inning, with second baseman
Alexi Amarista having been pinch–hit for the previous inning, Alonso moved to second base for the first time in his professional career. With left–handed hitter
Andre Ethier up after
Adrián González hit a ground ball to second that Alonso couldn't come up with, Alonso and third baseman
Jedd Gyorko switched positions. After Ethier was hit and two right-handed hitters were due, Alonso and Gyorko switched again, and the Padres got out of the inning after
A.J. Ellis grounded out to third. On June 6, Alonso was placed on the 15–day disabled list after he got hit on the hand by a pitch from
Aaron Loup on May 31.
Kyle Blanks and
Jesús Guzmán then entered a platoon while Alonso was out. At the time, he was hitting .284 with six home runs and 29 RBI. He was activated on July 12. On August 30, Alonso suffered a separate injury to his right hand, which caused him to miss most of September. Blanks and Guzmán filled in initially, but when
Tommy Medica was called up, he started the rest of the season at first. Alonso returned to action on September 28, and appeared in two games as a pinch–runner. In 97 games in 2013, he hit .281/.341/.368 with six home runs and 45 RBI. On June 19, 2014, Alonso was placed on the 15-day disabled list with right hand tendinitis. On July 26, after missing 30 games, Alonso was activated off the disabled list. After playing almost a month, Alonso was ruled out for the season on August 17 because of a forearm strain. In 84 games in 2014, Alonso hit .240/.285/.397 with 7 home runs and 27 RBI. In 2015, Alonso would spend time on the 15–day disabled list but would be activated on June 2. In 2015, Alonso hit .282 with five home runs and 31 RBI over 103 games; he had two separate stints on the disabled list because of a shoulder injury and strained lower back.
Oakland Athletics On December 2, 2015, the Padres traded Alonso and
Marc Rzepczynski to the
Oakland Athletics in exchange for
Drew Pomeranz,
José Torres, and a
player to be named later or cash considerations (later specified as
Jabari Blash). On January 13, 2016, the A's and Alonso announced that they had agreed to a one-year contract worth $2.65 million to avoid arbitration. Alonso avoided salary arbitration with the Athletics for a second consecutive season, by agreeing to a $4 million contract for the
2017 season on December 3, 2016. During the first half of the 2017 season, Alonso hit a career–high 20 home runs. He was named to the
2017 MLB All-Star Game. In 100 total games, he hit .267/.369/.527 with 22 home runs and 49 RBI.
Seattle Mariners On August 6, 2017, the Athletics traded Alonso to the
Seattle Mariners in exchange for
Boog Powell. In 42 games for Seattle, Alonso slashed .265/.353/.439 with six home runs and 18 RBI.
Cleveland Indians On December 20, 2017, Alonso signed a two–year contract with the
Cleveland Indians worth $16 million. The deal became official on December 23. In his first season in Cleveland, Alonso continued his power surge from the previous season, hitting 23 home runs with a career–high 83 RBI in 145 games. He had the lowest fielding percentage among major league first basemen, at .990.
Chicago White Sox On December 15, 2018, Cleveland traded Alonso to the
Chicago White Sox in exchange for minor league outfielder
Alex Call. On June 28, 2019, Alonso was
designated for assignment after he hit .178 with seven home runs and 27 RBI in 67 games. He was placed on release waivers on July 3.
Colorado Rockies On July 11, 2019, Alonso signed a minor league contract with the
Colorado Rockies. On July 23, the Rockies selected Alonso's contract. Alonso was mainly used as a pinch hitter for the Rockies, hitting .260 with three home runs and 10 RBI in 73 at–bats. On February 14, 2020, Alonso signed a minor league contract with the
Atlanta Braves. He was later outrighted off of the roster before the season began on July 23. On August 11, Alonso was traded to the
San Diego Padres in exchange for cash considerations. He did not play in a game in 2020 due to the cancellation of the minor league season because of the
COVID-19 pandemic. Alonso became a free agent on November 2. Alonso announced his retirement via
Instagram on November 20, 2020. ==Post-baseball career==