Kenly was born in Baltimore, Maryland. He studied law and was admitted to the bar in 1845, but went to the Mexican-American War as a lieutenant with a company of volunteers he had raised and was later promoted to the rank of major. He entered the American Civil War as colonel of the 1st Regiment Maryland Volunteer Infantry organized at Baltimore, Maryland, which was mustered into Union service on May 16, 1861. Together with some Pennsylvania companies, it was captured by Stonewall Jackson, after hard fighting, at Front Royal on the Shenandoah, May 23, 1862. Kenly himself was severely wounded when he was taken prisoner, but his stand had saved General Banks’s division at Winchester, and he was raised to the command of a brigade in 1862, which he led at Hagerstown, Harpers Ferry, and elsewhere.
Kenly joined the Army of the Potomac after the Battle of Gettysburg and was assigned to I Corps during the Bristoe Campaign and the Battle of Mine Run, commanding the third division of the corps. Afterwards, he was assigned to the Middle Department, commanding the Third Separate Brigade in 1864.
Kenly died in Baltimore, Maryland, and is buried there in Green Mount Cemetery.