Edward C. Mattox was born in December 1880, on his parents' homestead near Missouri Valley, Iowa, about twenty miles north of Council Bluffs. As a young man he worked briefly as a printers' devil on the Omaha Bee. He later worked for the Burlington railroad.

In August 1903, he arrived in Dietz, Wyoming, where he was hired as a telegraph operator and payroll clerk at a Megeath-owned coal mine. Mattox gradually moved up in the business until he was the general manager of all Megeath's Wyoming properties. In 1920, when the Megeaths sold their Dietz mine, Mattox was transferred to their Roundup, Montana, coal mine. This move represented a temporary set-back for Mattox, as he was only the assistant general manager at Roundup. However, he again moved up in the operation, becoming general manager in 1926, vice president in 1933, and finally president of the Roundup Coal Mining Company in the early 1940s. Dissatisfied with post-war conditions in the coal mining industry and in poor physical health, Mattox resigned from the company in July 1947.

In addition to his coal mining interests, Mattox owned the K7 Ranch, a half section of farm land just west of Bozeman, Montana. For many years, this ranch was operated for Mattox by Monte M. Benson. While working at the Dietz mine, Mattox married Ethel R. Adams. They had no children. Mrs. Mattox died in September 1944. Edward Mattox died January 6, 1949.