A farmer and dry goods store owner, he was a resident of Illinois when the Civil War broke out. In August 1861, Ellet was commissioned a captain in the 9th Missouri Volunteer Infantry Regiment, which later became the 59th Illinois Volunteer Infantry Regiment. In March 1862, he fought in the Battle of Pea Ridge. When his elder brother, Col. Charles Ellet, Jr., undertook the conversion of several river steamers to rams in the spring of 1862, Alfred Ellet became lieutenant colonel of Charles Ellet's U.S. Ram Fleet.
Following Charles Ellet's death in June 1862, Alfred took over the unit and was appointed brigadier general of the newly formed Mississippi Marine Brigade the following November. He commanded the Mississippi Marine Brigade during operations on the Western Rivers until 1864, when the unit was disestablished. He resigned his commission late in that year to return to civilian life.
Following the Civil War, Ellet was a businessman and civic leader in El Dorado, Kansas, where he died. He is buried there in Belle Vista Cemetery.
Alfred Washington Ellet, one of fourteen children, was born October 11, 1820, at Penn's Manor, Pennsylvania. He worked on his father's farm and studied civil engineering at Bristol Academy. In the years before the war he practiced his profession at various locations and on August 20, 1861, as a resident of Bunker Hill, Illinois, was commissioned a captain of the 59th Illinois Infantry. The following spring, when his brother, the celebrated engineer Charles Ellet, was ordered by the War Department to purchase vessels and convert them into rams, Alfred Ellet was commissioned an additional aide-de-camp with rank of lieutenant colonel and accompanied his brother who had been made colonel. They completed their fleet at Cincinnati and steamed down the river to Memphis, defeating the Confederate fleet there on June 6, 1862, and sinking or disabling eight of the nine enemy ironclads. Charles here received a wound which proved fatal fifteen days later, and the command devolved upon Alfred. With the Monarch and the Lancaster he steamed up the Yazoo and discovered and reported the presence of the Confederate ram Arkansas. Promoted to brigadier general of volunteers to rank from November 1, 1862, he was assigned to the Department of the Mississippi and placed in command of the Marine Brigade in 1863. After running the Vicksburg batteries in March, 1863, Ellet was engaged for some time in moving U. S. Grant's troops to the east bank of the Mississippi. In retaliation for information furnished the troops of Confederate J. R. Chalmers' command, Ellet burned Austin, Mississippi, which nearly resulted in the capture of one of his boats. He was on duty in New Orleans when he resigned his commission on December 31, 1864, to return to civil engineering. About 1868 he removed to El Dorado, Kansas, where he immediately became a prominent businessman and civic leader. With his son, Ellet organized one of the pioneer banks of the community and was instrumental in persuading an early railroad company (which is now part of the main line of the Santa Fe) to build through El Dorado. When he died there on January 9, 1895, his funeral "brought together the largest and most notable . . . concourse in the history of [El Dorado]." He was buried in the local cemetery.