Leiter was called up to the Major Leagues for the first time on April 18, 2017,[4] making his debut on April 28.[5] He finished his rookie season with a 3–6 record and a 4.96 ERA with 84 strikeouts over 27 appearances (11 starts).[6] He began 2018 on the disabled list and was optioned to Lehigh Valley after he was activated.
Toronto Blue Jays
On September 1, 2018, Leiter was claimed off waivers by the Toronto Blue Jays.[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. Leiter was designated for assignment by the Blue Jays on November 26.[8] He later cleared waivers and was assigned to the Triple–A Buffalo Bisons.
On March 17, 2019, the Blue Jays organization announced Leiter would miss the entire season after undergoing Tommy John surgery.[9] Leiter elected free agency following the season on November 4.[10]
Arizona Diamondbacks
On February 13, 2020, Leiter signed a minor league contract with the Arizona Diamondbacks. He did not play in a game in 2020 due to the cancellation of the minor league season because of the COVID-19 pandemic.[11] Leiter was released by the Diamondbacks organization on May 22.
On March 24, 2021, Leiter signed a minor league contract with the Detroit Tigers organization.[15] Leiter split the 2021 season with the Double-A Erie SeaWolves and the Triple-A Toledo Mud Hens. He made 25 appearances, going 10–8 with a 3.77 ERA and 145 strikeouts. He became a free agent following the season.
Chicago Cubs
On December 17, 2021, Leiter signed a minor league contract with the Chicago Cubs.[16] On April 16, 2022, Leiter's contract was selected by the Cubs.[17] On August 16, Leiter recorded his first career save in a game against the Washington Nationals.[18] In 35 appearances for Chicago, Leiter registered a 2–7 record and 3.99 ERA with 73 strikeouts in 67+2⁄3 innings pitched.
On January 13, 2023, Leiter was designated for assignment by Chicago after the signing of Eric Hosmer was made official.[19] On January 19, Leiter cleared waivers and was sent outright to the Triple–A Iowa Cubs.[20] However, four days later, Leiter rejected the outright assignment and elected free agency.[21] On February 2, Leiter re-signed with the Cubs on a minor league contract.[22] On March 30, the Cubs selected Leiter's contract, adding him to the major league roster.[23] In 69 relief appearances, he logged a 3.50 ERA with 77 strikeouts and four saves across 64+1⁄3 innings pitched.[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]
^Joyce, Greg. "Mark Leiter Jr. gets surprise call from IronPigs to Phillies", The Express-Times, April 19, 2017. Accessed May 10, 2017. "The Lanoka Harbor, N.J. resident split 2015 between Reading and High-A Clearwater – both starting and relieving – after splitting 2014 between Clearwater and Low-A Lakewood, only starting."
