After two decades of decline, however, the team left Philadelphia for Kansas City in 1955 and became the Kansas City Athletics.
Neverjetnih 50 let je na čelu ekipe bil njen prvi upravnik, Connie Mack, njen kader igralcev, ki so danes člani Hrama slavnih, pa je vključeval igralce, kot so bili Chief Bender, Frank "Home Run" Baker, Jimmie Foxx in Lefty Grove.