His great-great-uncle, Daniel D. Tompkins, was the sixth Vice President of the United States and, earlier, the Governor of New York. His grandfather, Colonel Daniel D. Tompkins, was an 1820 graduate of the United States Military Academy, and a veteran of the Mexican War and the Civil War. Tompkins's older brother was Colonel Selah "Tommy" Tompkins, a career officer with the 7th Cavalry who was its lieutenant colonel during the Punitive Expedition and led American forces during the Third Battle of Ciudad Juarez in 1919. His younger brother, also named Daniel D. Tompkins, became a colonel during World War I.