As a boy, Averell worked as a drugstore clerk in the nearby town of Bath, New York. He graduated from West Point in 1855, ranked 26 out of 34 in his class. He was commissioned a 2nd lieutenant in the U.S. Army Mounted Rifles. His early assignments included garrison duty at Jefferson Barracks, Missouri, and the U.S. Army Cavalry School in Carlisle, Pennsylvania. During 2 years of service in the southwestern United States, he was wounded in the Indians Wars in 1859 and was placed on the disabled list until the outbreak of the Civil War.
Averell was promoted to 1st lieutenant of the 3rd U.S. Cavalry on May 14, 1861. After the capture of Fort Sumter, he made a risky solo journey across the country to the New Mexico Territory with a message to summon his old mounted rifle regiment back east to join the fighting.
Averell first saw action at 1st Bull Run while acting as Assistant Adjutant General to Brigadier Gen. Andrew Porter. In August of 1861 he was appointed Colonel of the 3rd Pennsylvania Cavalry Regiment. From October 1861 through March 1862, he commanded a cavalry brigade protecting the Defenses of Washington, D.C. He led the cavalry through the Peninsula Campaign and the Seven Days Battles. Immediately after that campaign, on July 6, 1862, he was given command of the 1st Cavalry Brigade in the Army of the Potomac. He missed the battle of Antietam and most of the Maryland Campaign as he recovered from a bout of malaria that was known at the time as “Chickahominy Fever”. As Confederate cavalry Major Gen. J.E.B. Stuart rode around the Union Army and raided Chambersburg, Pennsylvania, he returned in time to lead his brigade in pursuit. After this, he was promoted to Brigadier General.
During the battle of Fredericksburg Averell commanded the Cavalry Brigade of the Center Grand Division of the Army of the Potomac. He ascended to division command—the 2nd division of the Cavalry Corps—on February 12, 1863. His division fought the first engagement in which Union cavalrymen claimed victory against their Confederate counterparts, the battle of Kelly's Ford. Averell received a brevet promotion to major in the Regular Army for his actions in this battle. But the 2nd Division's reputation was diminished as it participated in Major Gen. George Stoneman's fruitless cavalry raid at Chancellorsville 6 weeks later. On May 2, 1863, army commander Major Gen. Joseph Hooker relieved Averell of his command due to his slow performance during the raid. Hooker sent an angry telegram to the War Department informing them that Averell “seems to have contented himself between April 29 and May 4 with having marched ... 28 miles, meeting no enemy deserving of the name, and from that point reporting to me for instructions.”
Averell left the Army of the Potomac after his relief at Chancellorsville and fought a series of minor engagements in the Department of West Virginia at the brigade and division level. In November 1863, he conducted what is called Averell's West Virginia Raid against the Virginia and Tennessee Railroad. He received a brevet promotion to Lieutenant Colonel in the Regular Army for the battle of Droop Mountain in West Virginia, and to Colonel for actions during the Salem Expedition on December 15, 1863.
During the Valley Campaign of 1864 against Confederate Major Gen. Jubal A. Early, Averell fought under Major Gen. Philip Sheridan. He won a victory at Moorefield and was recommended for a brevet promotion to major general of volunteers, but the appointment was not confirmed. He was relieved of command a second time in his career on September 23, 1864, following a dispute with Major Gen. Philip H. Sheridan about Averell's actions at Fisher's Hill. It was for the lack of his aggressiveness in pursuing Early. This incident truly devastated him and he could not hide his misery. The Army bureaucracy generated a promotion for Averell before the end of the war, to brevet major general in the Regular Army on March 13, 1865.
After the war, Averell resigned from the army and accepted an appointment as U.S. Consul General to Canada from 1866–69. Later in life, his skill as an inventor of practical devices provided him with a handsome income. He bacame the president of a large manufacturing company. Among his inventions were methods for manufacturing steel castings and insulated electrical cable. He is most famous as the inventor of American asphalt pavement. He was the posthumous author of "Ten Years in the Saddle" in 1978 and posthumous co-author of "History of the Third Pennsylvania Cavalry, 60th Regiment Pennsylvania Volunteers" in 1905 . He was buried in Bath, New York.
1st Lieutenant - May 14, 1861
Colonel - August ??, 1861
Brigadier General - September 26, 1862
Major General - March 13, 1865
3rd Pennsylvania Cavalry Regiment
1st Cavalry Brigade in the Army of the Potomac
Cavalry Brigade/ Center Grand Division/ Army of the Potomac (December 1862)
2nd Cavalry Division (February 22- May, 1863)
4th Seperate Brigade/VIII Corps (May 16 1863- April 1864)
2nd Cavalry Division (April 26, 1864- May 18, 1865)