[7] The Cleveland Indians selected Brennan in the eighth round of the 2019 Major League Baseball draft.

[7] After signing with the team he was initially assigned to the Arizona League Indians before being promoted to the Class A Short-Season Mahoning Valley Scrappers.

[10] Brennan began the 2022 season with Akron and was promoted to the Triple-A Columbus Clippers after hitting .304 with 39 RBIs, which was leading the Eastern League.

Brennan hit his first major home run and triple in the same game off of Max Castillo on October 2.

[14] On May 22, 2023, Brennan accidentally hit and killed a bird with a batted ball during a game against the Chicago White Sox.