Early in his career, McCaw carried out assistant surgeon and surgeon assignments in the army's Department of the Missouri. In October 1887, he was assigned to duty at Fort Leavenworth, Kansas. In April 1888, McCaw was posted to Fort Crawford, Colorado. In August 1889, McCaw received promotion to captain and a surgeon's assignment at Fort McPherson, Georgia. In 1891, McCaw was reassigned to Camp Pilot Butte, near Rock Springs, Wyoming.
