Born in Foxcroft, Maine, Buck was graduated from Waterville College (now Colby College) in 1859. During the Civil War he entered the Union Army as captain of Company C, Thirteenth Regiment, Maine Volunteer Infantry. He was appointed lieutenant colonel of the Ninety-first United States Colored Troops in August 1863, was transferred to the Fifty-first United States Colored Troops in October 1864, and was made brevetted colonel of Volunteers for gallant conduct. He was mustered out of the service at Baton Rouge, Louisiana, in June 1866. He then served as delegate to the of Alabama in 1867, and as clerk of the circuit court of Mobile County in 1867 and 1868.