Up until the 2003-2004 school year, the PIAA consisted of 11 athletic districts across the Commonwealth of Pennsylvania. District 12 was established when the Philadelphia Public League joined the PIAA in 2003 after years of operating in a Philadelphia based City League for the majority of its tenure. Philadelphia Classical High School, a private high school in Philadelphia, would also join District 12 in 2004.

In 2008, the Philadelphia Catholic League joined suit and entered into District 12 combining with the Public League to form a more unified Philadelphia presence in the PIAA. 

in the early 2020s, more private schools would join up with District 12 including The City School in 2021 and Liguori Academy in 2022. As of now, District 12 has plans to add Girard College, Holy Ghost Preparatory School and The International Christian School as members starting at the beginning of the 2026 school year.