As a senior in 2021, Mayer batted .392 with 14 home runs and 45 RBIs.[9] In his final high schoolat bat, he hit agrand slam.[10] He was named theCollegiate Baseball National High School Player of the Year.[11] He was also named Mr. Baseball California State Player of the Year for the 2021 season by Cal-Hi Sports.[12]
TheBoston Red Sox selected Mayer in the first round, with the fourth overall selection, of the2021 Major League Baseball draft. It was the Red Sox's highest draft pick in over 50 years.[13] Several analysts had Mayer ranked as the top overall prospect in the draft and had described him as afive-tool player.[14] The pick gave the Red Sox a signing bonus allotment of $6,664,000.[15] Mayer officially signed with Boston on July 22, for the assigned slot price of $6.66 million.[16][17] Mayer reported to the club's spring training base atFort Myers, Florida, soon after signing.[18] He made his professional debut for theFlorida Complex League Red Sox on August 5,[19] drawing three walks.[20] His first two professional home runs came in back-to-back games in the second half of August.[21] Mayer finished his first professional season with 91at-bats in which he hit .275 with three home runs and 17 RBIs.[22]
Mayer was assigned to theSalem Red Sox to open the 2022 season.[23] In May 2022, he was ranked 14th in the list of baseball's top 100 prospects byBaseball America.[24] In early August, he was promoted to theGreenville Drive.[25] Over 91 games between both teams, heslashed .280/.399/.489 with 13 home runs, 53 RBIs, and 30 doubles along with going 17-for-17 instolen base attempts.[26] In January 2023, he was ranked 10th in theBaseball America list of top 100 prospects.[27] He was named theSouth Atlantic League Player of the Week for the first week of May, going 16-for-31 with three home runs, six doubles and 12 RBIs in one series.[28] After batting .290/.366/.890 in 35 games at Greenville in 2023, Mayer was promoted to theDouble-APortland Sea Dogs in late May.[29] He was the youngest Boston prospect promoted to Double-A since 2009.[30]
Mayer began the 2024 season with Portland, ranked as the Red Sox' top minor-league prospect byBaseball America.[31] In August, he was promoted to theTriple-AWorcester Red Sox, along with fellow-prospectsRoman Anthony andKyle Teel.[32] At the time of his promotion, Mayer had posted a .307/.370/.480 slash line with Portland.[32]
Mayer was promoted to the major leagues on May 24, 2025, for a game against theBaltimore Orioles atFenway Park.[33][34] Due to an injury toAlex Bregman, Mayer started at third base in his MLB debut game.[35] The following day, Mayer notched his first major-league hit, a single in the second inning, and his first extra-base hit, a double in the ninth inning.[36] On June 6, Mayer hit his first career home run offNew York Yankees pitcherWill Warren in the top of the fifth inning. It marked the first time that a Red Sox player recorded his first career home run atYankee Stadium since September 7, 2013, whenXander Bogaerts hit his first career home run, also at Yankee Stadium.[37] On June 11, Mayer hit two home runs against theTampa Bay Rays, becoming the youngest Red Sox player to have a multi-homer game sinceRafael Devers in 2018. He also became the third-youngest player in Red Sox history to have a multi-homer game within his first five career games, behind onlyBilly Conigliaro (21) andTed Williams (20).[38][39] In 44 appearances for Boston during his rookie campaign, Mayer batted .228/.272/.402 with four home runs and 10 RBI. On July 25, he was placed on the injured list due to a right wrist sprain.[40] On August 17, managerAlex Cora announced that Mayer would require season-ending wrist surgery.[41]
Mayer was raised in Chula Vista, and also has roots inAmbos Nogales where Mayer's family members have played baseball on both sides ofthe border. His parents, Enrique and Myriam Mayer, were both born inNogales Municipality, Sonora, Mexico, and Mayer is also related to Hector "Chero" Mayer who played in the minor leagues with theSt. Louis Cardinals organization in the 1950s.[46][47]
