He was commissioned assistant surgeon United States army May 28, 1861, and ordered to New Mexico, but upon his arrival in Missouri was attached to the array of General Lyon, was present when he was killed at Springfield, and subsequently received the thanks of the commanding general for bravery and skill in attendance upon the wounded.

Dr. Sprague was transferred to Assistant Surgeon General Wood's office, in St. Louis, where he remained until early in 1863, when he was placed in command of the Eliot General Hospital, in St. Louis. That was shortly discontinued, and he took charge of the hospital steamer " City of Memphis," transporting the sick and wounded of Grant's army around Vicksburg to hospitals up the river. During the final days of the siege of Vicksburg he displayed exalted bravery and fidelity in attention to the men torn with shot and shell, sent to his steamer for such aid as the surgeons could render them. In November he was ordered on duty as secretary of the Army Medical Examining Board, in New York City, and then to command of the McDougall General Hospital, at Fort Schuyler, New York Harbor. Thence he was returned to the Examining Board, and in May, 1865, resigned from the army, his name standing high on the list for promotion. He began the practice of medicine at West Farms, and in 1868 moved to Fordham. He was appointed health officer of the town of West Farms, was the first physician to the " Home for Incurables," and first physician to the " House of Rest for Consumptives," at Tremont. He was a member of the Westchester County Medical Society, president of the Y'onkers Medical Association, was elected a delegate to the American Medical Association for 1874 from the latter society, and was preparing to attend its meeting at Detroit, Mich., when he was arrested by death ; was a corresponding member of the American Microscopic Society, and member of the New York Pathological Society.