They became a professional team in 1920, joining the Southern League.[4] Both Millwall and Crystal Palace joined the Football League in the 1920–21 season,[3][4] playing in the Third Division, while Charlton Athletic joined the year after for the 1921–22 season,[4] finally competing at the same level as both their South London neighbours. Wimbledon became a Football League club five decades later in the 1977–78 season, playing their first South London derby against Millwall in 1980.