 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.Brennan lost to Dingell's son, John Dingell Jr.In 1953, he joined the law firm of Waldron, Brennan, Brennan, and Maher, with whom he worked until 1961, when he was elected to a seat on the Common Pleas Bench.