[2][3][4][5] Brennan joined the Atlanta United academy at the age of 12, just before the professional outfit began their inaugural Major League Soccer season.
[6] In October 2021, Brennan made his professional debut with Atlanta's USL Championship affiliate, starting and playing 60 minutes in a 4–2 defeat to Memphis 901.
In the 2023 American Family Insurance Cup against Liga MX side Toluca, Brennan scored coming off the bench.
[8] On March 9, 2023, Brennan signed his first professional contract with Atlanta United 2 who would be making their debut in MLS Next Pro.
[1] He made his MLS debut on June 21, 2023, coming on as a substitute for Amar Sejdić in the 87th minute in a match that ended 2–2 at home against New York City FC.