He enlisted as a Private in the 69th New York Militia. By 1859 he was appointed colonel of the regiment. The regiment was a state militia unit at that time composed of citizens, not soldiers, and was involved in the maintenance of public order. On October 11, 1860, Colonel Corcoran refused to march the regiment on parade for the 19-year-old Prince of Wales, who was visiting New York City at the time, protesting the British imposition of the Great Famine of Ireland. Corcoran was removed from command and a court martial was pending over that matter when the Civil War began.