During the Peninsula Campaign of 1862, Dennison commanded the rear (left) section of Tidball's "flying battery."

Dennison was promoted to the permanent rank of captain in 1867, and requested and received a discharge from the army in 1870.

He appears to have dabbled in politics, perhaps hoping to follow in his father's successful career as Governor of Ohio and Postmaster General of the United States.

His public reputation suffered due to unsavory business dealings with questionable acquaintances, and he lost the 1879 mayoral race in Columbus, Ohio.

He moved to Denver, Colorado, where he served as a district attorney and also dabbled in speculative mining and saloons.