The Pope John Lions compete in the Northwest Jersey Athletic Conference, which is comprised of public and private high schools in Morris, Sussex and Warren counties, and was established following a reorganization of sports leagues in Northern New Jersey by the New Jersey State Interscholastic Athletic Association (NJSIAA). Until the NJSIAA's 2009 realignment, the school had participated in the Sussex County Interscholastic League, which included public and private high schools located in Sussex County and northern Morris County. With 510 students in grades 10-12, the school was classified by the NJSIAA for the 2019–20 school year as Non-Public A for most athletic competition purposes, which included schools with an enrollment of 381 to 1,454 students in that grade range (equivalent to Group II for public schools). The football team competes in the United White division of the North Jersey Super Football Conference, which includes 112 schools competing in 20 divisions, making it the nation's biggest football-only high school sports league. The school was classified by the NJSIAA as Non-Public Group III for football for 2018–2020.