He was a law clerk in private practice in New York from 1915 to 1916.

He was an attorney for the New York State Tax Department.

[2] On March 31, 1942, President Franklin D. Roosevelt nominated Brennan to a seat on the United States District Court for the Northern District of New York vacated by Judge Frank Cooper.

Brennan was confirmed by the United States Senate on April 28, 1942, and received his commission on May 6, 1942.

He assumed senior status on May 1, 1963, and continued serving in that capacity until his death on April 9, 1968, in Utica, New York.