He was born in Frankfort, Kentucky, but the family moved to Mississippi while he was still a child. There he read law and was admitted to the bar. Moving to Louisiana, in 1861 he was appointed by Governor Thomas O. Moore to a military board organizing the state for war. He entered military service as a Lieutenant Colonel of the 1st Louisiana Regulars and was advanced to Colonel after the regiment was ordered to Pensacola, Florida. Early in 1862 he and his men were transferred to the West, where he remained for the duration of the war.

The son of Irish immigrants, he entered the United States Military Academy in 1841, graduating 25th in his class. Commissioned a 2nd Lieutenant in the 1st Dragoons-United States Regular Army, he served under Captain Philip Kearny in the Mexican War. On March 16, 1848, he was brevetted for gallantry and meritorious conduct at the Battle of Santa Cruz de Rosales and in 1851 was commissioned 1st Lieutenant. Promotion to Captain followed in 1856.