From 1812 until 1815, Beaumont served as an assistant surgeon in the Army during the War of 1812 participating in the Battle of Plattsburgh. After the war ended, he started a private practice in Plattsburgh, New York, but by 1820 Beaumont had rejoined the Army as a surgeon. He was assigned a location at Fort Mackinac. Beaumont took a leave in 1821, and married Deborah "Debby" Green Platt in Plattsburgh, before returning to his post. Deborah was divorced from Nathaniel Platt, whose uncle Zephaniah Platt founded Plattsburgh after the beginning of the Revolutionary War. Her father, Israel Green, was a third cousin of General Nathanael Greene.