Chattanooga Campaign
When Ulysses S. Grant became commander of the Military Division of the Mississippi, Parker became his adjutant during the Chattanooga Campaign. He was subsequently transferred with Grant as the adjutant of the U.S. Army headquarters and served Grant through the Overland Campaign and the Siege of Petersburg. At Petersburg, Parker was appointed as the military secretary to Grant, with the rank of lieutenant colonel. He wrote much of Grant's correspondence.