Earl attended the Fort Edward Institute. When he was 19, he started working for the "Hudson River Chronicle", later becoming its publisher. In July 1861, after the American Civil War began, he enlisted in the 40th New York Infantry Regiment and was mustered in as Sergeant Major. The regiment was initially sent to Alexandria, Virginia and guarded it during the First Battle of Bull Run, He was then sent to join George B. McClellan's army. He participated in the Siege of Yorktown and the Battle of Williamsburg. In May 1862, after the latter battle, he was promoted to Second Lieutenant. He then participated in the Battle of Seven Pines. He accompanied his regiment while retreating through the Seven Days Battles, even though he was suffering from swamp fever. In July 1862, he was discharged due to his illness. In December 1862, he re-enlisted as First Sergeant in the 176th New York Infantry Regiment and participated in the Banks expedition to New Orleans. In April 1863, while stationed in Thibodaux, Louisiana, he was permanently disabled due to a bad fracture in his right ankle bone. In June 1863, while still crippled, he participated in a skirmish in Thibodaux as a mounted force and fought in the Battle of LaFourche Crossing. In September 1863, he was honorably discharged due to his disability. While serving in Louisiana, he started a small union paper in Thibodaux and acted as an occasional correspondent of the "New York Sunday Mercury".