In 1914, Southworth married Lida Brooks. She was a minister's daughter and they had met while Southworth was playing for Portsmouth. The couple's son, William Brooks Southworth, was born during Southworth's early playing career. Billy Southworth Jr. later became a professional baseball player for several seasons. The elder Southworth returned to the Cleveland Indians in 1915 and appeared in 60 games. He played for the Birmingham Barons in 1917 and part of 1918, when he made the Pittsburgh Pirates and played in 64 major league games.