William A. Mellen was born around 1841 in Tompkins County, New York, to Philip and Rachel Mellen. His father was a farmer who owned $300 of personal property in 1860. The family moved to Dekalb County, Illinois, sometime in the 1850s, and Mellen worked as a laborer on his father’s farm.
He enlisted in the Union army on August 12, 1862, and mustered in as a private in Company C of the 74th Illinois Infantry on September 4, 1862. According to his service records, he was 6 feet, 1 inch tall, with brown hair and black eyes. The regiment took part in the Battle of Perryville, the Battle of Stones River, and the Chattanooga campaign. He remained devoted to the Union, writing in June 1863 that “I have never regreted [sic] that I enlisted to fight for the union[.] I may never survive to return home but if I fall on the field of battle I shall have the consolation of feeling that I could never die in a better cause.” He died in Georgia on May 23, 1864.