The family moved to Atlanta in 1852 and, at the age of sixteen, William began reading law at his father's firm.

In March 1862, Calhoun enlisted in the Confederate States Army and was commissioned as captain of Company K of the 42nd Georgia Infantry.

He served in Knoxville, Tennessee, and then in the Vicksburg Campaign, where he and his regiment were surrendered by John C. Pemberton on July 4, 1863.

After being exchanged, Calhoun served in the Atlanta Campaign and was wounded at the Battle of Resaca.

He left his law practice to serve as judge of the Court of Ordinary of Fulton County from 1881 to 1897. a — acting Mayor