Robert A. McCabe, an officer in the United States Army and a subagent for the Bureau of Indian Affairs, was born in Pennsylvania. In Bryseville Kentucky, 1808, he was recruited by Captain Thornton Posey of the 7th Infantry. McCabe believed he would be appointed a cadet but learned that Captain Posey was not qualified to designate cadets. He received no compensation for his first 11 months in the infantry but decided to remain in the army. In January 1812, he became an ensign in the First Infantry. He received several promotions and, in May 1824, he was promoted to captain.

In 1826, McCabe was assigned to the Fifth Infantry and was involved with the planning of a garrison on a new site at Prairie du Chien, Wisconsin. Apparently, McCabe had received sufficient education to prepare architectural drawings and to determine precise dimensions for work on the proposed buildings.

In 1829, McCabe was posted at Fort Mackinac, Mackinac Island, Michigan, an area populated by the Objibway Indians of the Chippewa tribe. It is likely that McCabe, as an army officer, represented government interests in the absence of an authorized agent from the Bureau of Indian Affairs. His attention to Indian affairs may have motivated him to compile a dictionary of the Objibwa (Chippewa) language included in his letterbook. McCabe resigned from the army in 1833. In the same year, he may have acted as a recorder when a compilation of laws was written down and agreed upon for the Muscogee (Creek) Nation.

In 1833, McCabe was employed by the Bureau of Indian Affairs, Prairie du Chien Agency. He served as subagent at Fort Winnebago which was established for the Winnebago Indians living in the vicinity of the Fox and Wisconsin Rivers. In 1834, he was assigned to the Western Creek Agency whose headquarters was located at various places in the vicinity of Fort Gibson (Oklahoma) and the confluence of the Arkansas, Grand, and Verdigris Rivers. At Fort Gibson, he was responsible for the Creek Indians.

McCabe's health deteriorated and he suffered from a paralysis which affected his right hand. In 1835, he resigned from the Bureau of Indian Affairs and expressed his intention to visit Cameleon Spring, Kentucky to restore his health. He then settled in Blairsville, Indiana County, Pennsylvania.

A biographical note at the front of McCabe's letterbook in a different handwriting, describes McCabe as a sutler to Fort Mackinac, 1836 to 1840. McCabe began an autobiographical sketch in September 1838. He was writing at that time from Blairsville, Pennsylvania and his handwriting reflects the paralysis from which he was suffering. It is unknown when he died.