In December 1955, he was the Republican Party candidate in the special election for the U.S. representative from Michigan's 15th congressional district to fill the seat of John Dingell Sr., who had died in office.

In 1963, he was appointed by Michigan Governor George W. Romney to the Wayne County Circuit Bench, and in 1964 he was elected to that same position.

In 1966, at the urging of Governor Romney, Brennan, sought the nomination of the Republican Party as Associate Justice of the Michigan Supreme Court.

Brennan left the Supreme Court on December 6, 1973, to dedicate his professional career to the newly formed law school.

In 1982, he was an unsuccessful candidate for Lieutenant Governor of Michigan, losing to Democrat Martha W. Griffiths in the general election.