Brennan was born at Urbana, Champaign County, Ohio, in 1864, and attended public and parochial schools in that city.

At age 21, he was nominated for recorder of Champaign County by the Democratic Party.

He was a member of the Democratic Central Committee for ten years, and chairman of the Champaign Democratic Executive Committee for ten years.

He was a member of Urbana City Council for ten years.

He won election for Treasurer in 1912, and began his term January, 1913.