Emigrating from his native Scotland, Dempster enlisted in the United States Navy for a three-year term in August 1864 and was assigned to the Union ironclad United States Ship. His enlistment is credited to the state of Pennsylvania. On January 15, 1865, the North Carolina Confederate stronghold of Fort Fisher was taken by a combined Union storming party of sailors, marines, and soldiers under the command of Admiral David Dixon Porter and General Alfred Terry.
He received the Medal of Honor for his actions during the Second Battle of Fort Fisher on January 15, 1865. The President of the United States of America, in the name of Congress, takes pleasure in presenting the Medal of Honor to Coxswain John Dempster, United States Navy, for extraordinary heroism in action while serving on board the United States.S. New Ironsides during action in several attacks on Fort Fisher, North Carolina, 24 and 25 December 1864. And 13, 14, and 15 January 1865.
When the enemy came out of his bombproofs to defend the fort against the storming party, the ship"s battery disabled nearly every gun on the fort facing the shore before the cease-fire orders were given by the flagship.
