He graduated from the United States Military Academy in 1860 and was commissioned a second lieutenant in the 2nd Dragoons, serving initially in Utah under John Buford.

He received a brevet promotion to lieutenant colonel in the regular army for his actions at the Battle of Yellow Tavern, the engagement in which Confederate cavalry commander Maj. Gen. J.E.B.

Arriving at the opportune moment, his division routed the Confederate forces at the Third Battle of Winchester, a deed for which he received a brevet promotion to major general of the volunteers.

October 5 his division alone destroyed from Port Republic to Toms Brook 630 barns, 47 mills, 410,742 bushels of wheat, 515 acres of corn not counting private homes.

He was brevetted major general in the regular army, in April 1865, for bravery at the Battle of Five Forks and the Appomattox Campaign.

On January 28, 1866, Merritt was one of a number of brevetted generals mustered out of volunteer service and returned to their pre-war ranks in the regular army.

He was made colonel of the 5th Cavalry on July 1, 1876, which he commanded in the Battle of Slim Buttes during the American Indian Wars.

Merritt and Dewey made arrangements with Governor General Fermín Jáudenes, commander of the Spanish garrison, to surrender the city to the American forces after the latter put up a token resistance.

Merritt was relieved by Major General Elwell Stephen Otis on August 30 to advise the United States delegation in the peace negotiations leading to the Treaty of Paris.

General Merritt retired from the Army in 1900 and died from complications of arteriosclerosis in Natural Bridge, Virginia, at the age of 74, on December 3, 1910.