After garrison duty in the harbor of New York in 1812–13, he was ordered to the Niagara frontier in 1813 for service in the War of 1812.
On March 9, 1851, Loomis was promoted to colonel and was given command of the 5th U.S. Infantry Regiment at Fort Belknap, Texas (1852–1853).
He remained on active duty after the war to process administrative and court martial proceedings.
In 1867 he was still in uniform, managing army supply, personnel, and administrative issues related to his duties as a court martial officer.
[2] On July 17, 1866, President Andrew Johnson nominated Loomis for appointment to the brevet grade of brigadier general in the Regular Army for long and faithful service, to rank from March 13, 1865, and the U.S. Senate confirmed the appointment on July 26, 1866.