At the age of twenty, Gillmore was appointed to United States Military Academy at West Point. He graduated in 1849, head of his class and was commissioned as an engineer officer. Prior to the Civil War his most significant area of service was as an engineering instructor at West Point. His mathematical and engineering background would serve him well in the early part of the Civil War. Gillmore performed well at the tactical level of command. In two separate tactical actions during the Civil War, forces under General Gillmore achieved victories for the Union Army. In April 1862, he commanded the forces that caused the surrender of Fort Pulaski, Georgia. This victory closed Savannah as a Confederate blockade running port. It also was the first time rifled artillery was used against masonry forts. This operation was anticipated to take several weeks, but Gillmore's force defeated the Fort within 30 hours. The rifled artillery was very effective against Fort Pulaski, rendering masonry forts almost obsolete from that point forward. Gillmore received praise and honor for the victory. Beginning the siege as only a captain, he finished with the rank of Brigadier General of Volunteers.