John Ranahan was born in 1836 in County of Monahan, Ireland, and later immigrated to the United States. He enlisted in the U.S. Marine Corps on June 21, 1861, in Philadelphia, Pennsylvania, and served aboard the steam frigate USS Minnesota during the U.S. Civil War. He was awarded the Medal of Honor on June 22, 1865, for action aboard the USS Minnesota on January 15, 1865, during the Second Battle of Fort Fisher. Corporal Ranahan was honorably discharged from the U.S. Marine Corps on January 24, 1865, and died on June 1, 1892. John Ranahan is buried at the St. Martin Catholic Church Cemetery in New Derry, Pennsylvania.

His Medal of Honor citation reads:

On board the U.S.S. MINNESOTA in the assault on Fort Fisher, 15 January 1865. Landing on the beach with the assaulting party from the ship, Corporal Rannahan advanced to the top of the sandhill and partly through the breach in the palisades despite enemy fire which killed or wounded many officers and men. When more than two-thirds of the men became seized with panic and retreated on the run, he remained with the party until dark when it came safely away, brining its wounded, its arms and its colors.

John Ranahan's last named was misspelled "Rannahan" on both his Medal of Honor citation and the General Orders for his Medal of Honor.