 A native of Milwaukee, Wisconsin and a standout multi-sport athlete at Marquette University High School, Brennan played halfback at Notre Dame from 1945 to 1948, graduating in 1949.After graduating from Notre Dame, Brennan coached at Mount Carmel High School in Chicago and won three successive city championships.Brennan returned to Notre Dame in 1953 as freshman football coach and succeeded Frank Leahy as head coach the following year.