He was also a Harvard graduate (1852), lawyer and legal historian, and Union soldier in the American Civil War, during which he was wounded, captured, imprisoned, and exchanged.
Shortly after the attack on Fort Sumter, Quincy was commissioned a captain in the 2nd Massachusetts Infantry Regiment on May 25, 1861. He was promoted to major on October 22, 1862, and to colonel on January 18, 1863. He resigned his commission on June 5, 1863, but was re-commissioned as the lieutenant colonel of the 73rd United States Colored Infantry Regiment on November 29, 1863, and was promoted to colonel in command of the regiment on May 29, 1864. He served briefly as Mayor of New Orleans from May 5 to June 8, 1865.
He transferred to the 96th US Colored Infantry Regiment on September 27, 1865, and was mustered out on January 21, 1866, and became the colonel of the 81st US Colored Infantry the next day. He was honorably mustered out of service on November 30, 1866.[3]