When the 2020 Minor League Baseball season was canceled due to the COVID-19 pandemic, Marsh was invited to continue his development at an alternate training site.

Marsh made the Angels' Opening Day roster in 2022 as part of a platoon of corner outfielders, but he was traded to the Philadelphia Phillies partway through the season.

He and teammate Joey Bart, a future Major League Baseball (MLB) catcher, took Buford to a Georgia High School Association state championship title.

[15][16] He struggled with the transition to the California League, going 3-for-41 in his first 10 games,[13] and at the end of May, hitting coordinators worked with Marsh to shorten his swing and improve his pitch contact.

[19][20] He finished the season batting .300 with seven home runs and 43 RBI in 96 games,[15] and he received both Mid- and Post-Season All-Star honors from the Southern League.

[21] Later that year, Marsh played for the Mesa Solar Sox in the Arizona Fall League, where he batted .328 with two home runs and 11 RBI in 19 games.

[22] Early in the Angels' 2020 spring training, Marsh suffered a Grade 2 strain in his left elbow while diving to make a catch in center field.

As a result, high-ranked prospects, including Marsh, were assigned to alternate training sites to continue their development and await potential major league promotions.

[25] Working out of Blair Field, Marsh spent the season developing his hitting and learning how to play first base as a supplement to his usual outfield positions.

[29][30] The injury delayed his spring conditioning, and Marsh did not make his season debut until May 13, when he joined the Triple-A Salt Lake Bees in the Pacific Coast League.

[37] The following day, Marsh recorded his first three major league hits, as well as an RBI on a ninth inning double, as Los Angeles lost 4–1 to the Oakland Athletics.

[44] Manager Joe Maddon planned to use Trout in center field while rotating Marsh, Adell, and Taylor Ward in the corner outfield positions.

[49] Philadelphia, who had lacked a long-term center field option, had targeted Marsh during the previous offseason, but were unable to acquire him until the MLB trading deadline.

[50] On August 17, during a game against the Cincinnati Reds, Marsh suffered a bone bruise in his knee and injured his ankle when he ran into the outfield wall at Great American Ball Park.

[54] For 2023, Marsh was the everyday center fielder against right-handed pitching and improved his offensive production, batting .277/.372/.458 and practically doubling his walk total to 59, with 144 strikeouts.