The
Boston Red Sox selected Mayer in the first round, with the fourth overall selection, of the
2021 Major League Baseball draft. It was the Red Sox's highest draft pick in over 50 years. Several analysts had Mayer ranked as the top overall prospect in the draft and had described him as a
five-tool player. The pick gave the Red Sox a signing bonus allotment of $6,664,000. Mayer officially signed with Boston on July 22, for the assigned slot price of $6.66 million. Mayer reported to the club's spring training base at
Fort Myers, Florida, soon after signing. He made his professional debut for the
Florida Complex League Red Sox on August 5, drawing three walks. His first two professional home runs came in back-to-back games in the second half of August. Mayer finished his first professional season with 91
at-bats in which he hit .275 with three home runs and 17 RBIs. Mayer was assigned to the
Salem Red Sox to open the 2022 season. In May 2022, he was ranked 14th in the list of baseball's top 100 prospects by
Baseball America. In early August, he was promoted to the
Greenville Drive. Over 91 games between both teams, he
slashed .280/.399/.489 with 13 home runs, 53 RBIs, and 30 doubles along with going 17-for-17 in
stolen base attempts. In January 2023, he was ranked 10th in the
Baseball America list of top 100 prospects. He was named the
South 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. After batting .290/.366/.890 in 35 games at Greenville in 2023, Mayer was promoted to the
Double-A Portland Sea Dogs in late May. He was the youngest Boston prospect promoted to Double-A since 2009. Mayer began the 2024 season with Portland, ranked as the Red Sox' top minor-league prospect by
Baseball America. In August, he was promoted to the
Triple-A Worcester Red Sox, along with fellow-prospects
Roman Anthony and
Kyle Teel. At the time of his promotion, Mayer had posted a .307/.370/.480 slash line with Portland. Due to an injury to
Alex Bregman, Mayer started at third base in his MLB debut game. 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. On June 6, Mayer hit his first career home run off
New York Yankees pitcher
Will Warren in the top of the fifth inning. It marked the first time that a Red Sox player recorded his first career home run at
Yankee Stadium since September 7, 2013, when
Xander Bogaerts hit his first career home run, also at Yankee Stadium. On June 11, Mayer hit two home runs against the
Tampa Bay Rays, becoming the youngest Red Sox player to have a multi-homer game since
Rafael 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 only
Billy Conigliaro (21) and
Ted Williams (20). 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. On August 17, manager
Alex Cora announced that Mayer would require season-ending wrist surgery. ==Personal life==