Career
He was one of the owners of the Portage Daily Register. He served on the Columbia County, Wisconsin Board of Supervisors, the Portage Common Council, and was city marshal. Brannan served as Mayor of Portage in 1872 and 1873.
He then served in the Wisconsin State Assembly in 1873 as a Republican.
Then, Brannan served as postmaster of Portage, Wisconsin until his death. He died in Portage, Wisconsin.