On July 2, 1964, he was appointed by Mayor Robert F. Wagner, Jr. to the New York City Criminal Court.
In November 1968, he was elected to the New York City Civil Court and returned to the bench in January 1969.
On July 25, 1985, he was indicted by a federal grand jury for taking $47,000 in bribes to fix cases in his court.
[7] Brennan served 26 months of his sentence, and was released on parole from Allenwood Federal Prison on May 13, 1988.
On the same day, his appeal was judged: nine of the original 26 charges were dismissed, and his fine was reduced to $9,000, but the prison sentence was upheld.