Harry Hamlet was commissioned in 1896 through the School of Instruction of the Revenue Cutter Service. From 1928 to 1932 he served as Superintendent of the Coast Guard Academy. From 1932 to 1936 he served as the 7th Commandant of the Coast Guard. He retired as a U.S. Coast Guard Rear Admiral in 1938, but was promoted one grade to Vice Admiral by virtue of his having served more than 40 years in the Coast Guard.