In 1925, he graduated with a law degree from Fordham University and practiced law following his admission to the bar the next year.
He subsequently served as assistant district attorney in Brooklyn, New York, from 1940 to 1944. In 1944, Rooney was elected by special election to the 78th United States Congress, to fill the vacancy left after the death of Thomas H. Cullen. He was re-elected in each subsequent election until opting to retire after the 1974 midterm election.
He resigned from his seat on December 31, 1974, a few days before his term was to expire.
Rooney died on October 26, 1975 in Washington, District of Columbia