 He was mustered out of the volunteers on January 15, 1866.Willcox returned to Detroit to resume his career as a lawyer but when the United States (Regular Army) was expanded in July 1866, he accepted appointment as colonel of the 29th U.S. Infantry Regiment.On March 26, 1867, President Andrew Johnson nominated Willcox for appointment to the brevet grade of brigadier general, USA (regular army), to rank from March 2, 1867, and the U.S. Senate confirmed the appointment on April 5, 1867.