He first enlisted April 21, 1861, as a corporal in Company G, 23rd Pennsylvania Infantry (a 3-month regiment) and mustered out July 31, 1861, at Philadelphia.

He was subsequently promoted to captain on August 29, 1863, and mustered out on the same day to accept a transfer to command Company A, 8th United States Colored Infantry.

Almost immediately after the Civil War, Beath became active in establishing a veterans' organization.

Beath is credited with writing the first history of the organization (1888), establishing its rules and regulations, and was a member of the board of trustees for the permanent fund.

He was also a veteran companion of the Pennsylvania Commandery of the Military Order of the Loyal Legion of the United States.