During the American Civil War, he participated in the Battle of Mobile Bay in 1864. He was promoted to captain in 1884 and to rear admiral in 1898. He commanded the European squadron and the North Patrol squadron in 1898.
Rear Admiral Howell wrote the first truly scientific American work on the deviations of the compass, patented a disappearing gun carriage, and originated the gyroscopic steering torpedo. He designed and patented the "Howell torpedo."
Howell retired from the Navy in 1902.