Mack was the son of a farmer and educated in the public schools of Indiana County, Pennsylvania and at a business college in Johnstown.
"Seph" Mack began his career as a stock room clerk at McCrory Stores in Johnstown, Pennsylvania, which was owned by his cousin, J. G. McCory.
Murphy Co., McKeesport, Pennsylvania, which owned about a dozen “five and dime” stores near Pittsburgh but which had begun to fail after the premature death of its founder in 1909.
Employee benefits included free life insurance, two weeks paid vacation, and Christmas bonuses of up to $15 in gold.
"[4] Mack enjoyed maintaining and enlarging his family farm, Old Home Manor, and frequently spent his weekends tending show horses and Black Angus cattle.