As a true freshman, Mitchell was named the Bruins starting right fielder and the Preseason Freshman of the Year by Baseball America.[10] Over 44 games, he batted .280 with 44 hits and 31 RBIs, missing time during the season due to health issues.[11][12] Mitchell played summer baseball in the Northwoods League for the Mankato MoonDogs.[13] During his sophomore year in 2019, he batted .349 with 14 doubles, 12 triples (a UCLA single season record), six home runs, 57 runs scored, and 41 RBIs and was named first team All-Pac-12 Conference and a third team All-American by the NCBWA and ABCA.[14][15] Mitchell spent the following summer playing for the United States collegiate national baseball team.[16]
The Milwaukee Brewers selected Mitchell in the first round, with the twentieth overall pick, in the 2020 Major League Baseball draft.[20] He signed with the Brewers on July 7 and received a $3.2 million bonus.[21] Mitchell was named to Milwaukee's 2021 Spring Training roster as a non-roster invitee and batted .367 with one home run and six RBI in 22 games.[22][23]
Mitchell was assigned to the High-AWisconsin Timber Rattlers to start the 2021 season.[23] He was promoted to the Double-ABiloxi Shuckers in early July after slashing .359/.504/.620 with five home runs, 33 runs scored, 20 RBI, and 12 stolen bases over 28 games with the Timber Rattlers.[24] Over 35 games with Biloxi to end the season, he batted .186/.291/.264 with three home runs and ten RBI over 35 games.[25] He missed time during the season due to a leg injury.[26]
Mitchell returned to Biloxi to start the 2022 season.[27] He suffered an oblique injury in May and was placed on the injured list until July 7, 2022, after a rehab assignment with the Arizona Complex League Brewers.[28] Mitchell batted .277 with four home runs and 25 RBI in 44 games with the Shuckers before being promoted to the Triple-ANashville Sounds.[29]
On August 27, 2022, the Brewers selected Mitchell's contract from Nashville, promoting him to the major leagues.[30] He missed the phone call from Sounds manager Rick Sweet informing him of his promotion to the majors, telling reporters it was because he was eating a chocolate chip cookie.[31] He made his debut later that day, playing center field at the end of the game.[32] He had his first major league start the day after, on August 28, playing center field and batting 9th. His first at-bat resulted in a flyout to left field.[33]
His second at-bat was his first major league hit, a two RBI double into centerfield that gave Milwaukee the lead. He hit his first home run against Wil Crowe of the Pittsburgh Pirates in the bottom of the eighth inning on August 29, 2022.[34] Mitchell completed the 2022 season batting .311/.373/.459 with 19 hits, 2 home runs, and 9 RBI, all in 28 games.[35]
Mitchell made the opening day roster for the Brewers, and also made the starting lineup, batting 7th and playing center field. On April 2, Mitchell hit his first MLB career triple, a sharp line drive to right field, driving in Brian Anderson.[36] Two days later, in a game against the New York Mets, Garrett had his first career two home-run game.[37][38] On May 4, Mitchell underwent shoulder surgery to repair the labrum in his left shoulder. According to manager Craig Counsell, the procedure was likely to end Mitchell's season.[39] On September 28, Mitchell was activated from the injured list.[40] He played in 19 total games for Milwaukee in 2023, hitting .246/.315/.446 with three home runs and seven RBI.
In 2024, Mitchell suffered a broken left hand in spring training and was placed on the injured list to begin the season.[41] He was transferred to the 60–day injured list on April 14, 2024.[42] Mitchell was activated from the injured list on July 1.[43]
Later that year, his most memorable MLB moment occurred on October 2nd when his two-run home run with two outs in the bottom of 8th carried the Brewers to a 5-3 victory over the New York Mets in Game #2 of the Wildcard series.