He would subsequently serve in the United States Army in various capacities over a period of forty years.

He was wounded and captured in the First Battle of Bull Run (First Manassas) while in command of a brigade in Maj. Gen. Samuel P. Heintzelman's division.

[1] After his release and exchange more than a year later, on August 19, 1862, President Abraham Lincoln appointed Willcox a brigadier general of volunteers, to rank from July 1, 1861.

[4] 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.

[5] 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.

[7] Willcox transferred to the 12th U.S. Infantry Regiment in 1869 and served in San Francisco, except for brief periods, until 1878 when he became Commander of the Department of Arizona.

[4] General Willcox was a member of the District of Columbia Commandery of the Military Order of the Loyal Legion of the United States and the Society of the Sons of the American Revolution.

[4] He died in Cobourg, Ontario, at 84 years of age but was buried in Section 1, Grave 18, of Arlington National Cemetery in Virginia.