Arnold was born in Gainesville, Florida, the first of five children of U.S. Army Major Albert C. "AC" Arnold and the former Irene Dominick. A far-ranging adventurer, AC Arnold had fought in the Boer War on the side of the Boers; joined the Seventh Cavalry as a trooper; been a riverboat gambler; fought beside Brigadier General John J. Pershing on the Mexico Punitive Expedition; been awarded a Distinguished Service Cross for actions with the 1/326 Infantry at Château-Thierry during World War I; gone to law school; and been assigned to several positions in the peacetime Army before rejoining the Seventh at Fort Lewis, Washington, where he died in 1932.