David McIntosh was practicing law when the Civil War broke out in 1861. The only military experience he had was as a member of the local militia before the war, but after his native South Carolina seceded on December 20, 1860, McIntosh offered his services to the state. On July 29, 1861 he was appointed captain of Company D, 1st South Carolina Infantry, seeing action at the Battle of Vienna. His company was converted to the Pee Dee Light Artillery in 1862 and he saw action on the Peninsula Campaign, the Battle of Harpers Ferry, the Battle of Antietam, and the Battle of Fredericksburg.
On March 2, 1863, McIntosh was promoted to major and given command of an artillery battalion. He commanded his battalion at theBattle of Chancellorsville, the Gettysburg, Bristoe Station, and the Mine Run. McIntosh was promoted to lieutenant-colonel in February 1864 and commanded his battalion in the Overland Campaign, including the Battle of the Wilderness. He fought along the siege lines at the Siege of Petersburg and was slightly wounded at the Battle of the Crater. Shortly thereafter he was wounded at the Battle of Weldon Railroad. McIntosh was present with the battalion until just before Appomattox where he disappears from the record.
McIntosh was a brother-in-law to Confederate General John Pegram and his younger brother William J. Pegram. William Pegram was also a famous "gunner" in the Third Corps of the Army of Northern Virginia with McIntosh. McIntosh was married to Virginia Pegram.
When the War ended, McIntosh resumed the practice of law, this time in Towson, Maryland. Eventually McIntosh becoming the head of the Maryland state bar association. His notable post-war accomplishments include authoring a pamphlet on the battle of Chancellorsville.