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.

As a captain he led a company in the Baltimore-Washington Battalion and wrote a book about his experiences, Memoirs of a Maryland Volunteer.

[2] During the Battle of Monterrey on September 21–24, 1846, Kenly's battalion was involved in heavy fighting and Colonel William H. Watson was killed.

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.