He has previously played in MLB for the Philadelphia Phillies, Toronto Blue Jays, and Chicago Cubs.

He was selected by the Phillies in the 22nd round of the 2013 Major League Baseball draft, and made his MLB debut with them in 2017.

He made his professional debut that year with the GCL Phillies and he was promoted to the Clearwater Threshers and Lakewood BlueClaws during the season.

[7] He was activated from the disabled list on September 3 and made 8 appearances for Toronto, struggling to a 13.50 ERA with 9 strikeouts across 6+2⁄3 innings.

On March 17, 2019, the Blue Jays organization announced Leiter would miss the entire season after undergoing Tommy John surgery.

[15] Leiter split the 2021 season with the Double-A Erie SeaWolves and the Triple-A Toledo Mud Hens.

On January 13, 2023, Leiter was designated for assignment by Chicago after the signing of Eric Hosmer was made official.

[24] Leiter made 39 appearances out of the bullpen for the Cubs in 2024, compiling a 2–4 record and 4.21 ERA with 53 strikeouts over 36+1⁄3 innings pitched.

[25] On July 30, 2024, the Cubs traded Leiter Jr. to the New York Yankees in exchange for infielder Ben Cowles and pitcher Jack Neely.