Mackie enlisted in the Marine Corps from New York on April 24, 1861. His first assignment was with the Marine Detachment on board the USS "Savannah". He was promoted to corporal on March 1, 1862. On April 1, he began serving on the ironclad warship USS "Galena". On May 15, a five-ship Union Navy squadron, including the "Galena", steamed up the James River to test the defenses of the Confederate capital, Richmond, Virginia. After reaching a bend in the river upstream of Dutch Gap, the squadron encountered submerged obstacles and heavy fire from two battalions of Confederate Marines positioned on the banks of the river and artillery fire from Fort Darling atop Drewry's Bluff about eight miles below Richmond. Mackie commanded 12 Marines on the gun deck. The fort's artillery batteries inflicted severe damage on the "Galena" and forced the Union squadron to turn back. During the battle (which would come to be known as the Battle of Drewry's Bluff), most of "Galena"'s naval gun crew was killed or wounded. Mackie led a group of Marines who took over operation of the guns for the remainder of the battle. On November 1, he was promoted to orderly sergeant. In June 1863, he was assigned to the .