The Mount St. Mary Academy Mount Lions compete in the
Skyland Conference, which is comprised of public and private high schools spanning
Hunterdon, Somerset and
Warren counties in northern
New Jersey, operating under the jurisdiction of the
New Jersey State Interscholastic Athletic Association (NJSIAA). Prior to the NJSIAA's 2009 realignment, the school had participated in the
Mountain Valley Conference, which included public and private high schools in
Essex County, Somerset County and
Union County. With 528 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 swimming team won the Non-Public Group B state championship in 2003, 2004, 2007-2009 and 2011, and won the Non-Public A title in 2005 (as co-champion with
Immaculate Heart Academy) and 2006. The program's seven state titles are tied for fourth-most in the state. The swim team won the Non Public B state championship in 2007 with a 116–54 win vs.
Pingry School. In the 2013 finals at
The College of New Jersey, the team defeated defending champion
Mount Saint Dominic Academy 95–75 to win the Non-Public B title. In 2020 and 2022, the team became the Raritan Division Skyland Conference Champions while also going on to become the NJSIAA A Division Champions and the NJSIAA Non-public “A” State Champions for the 2022 season. The cross country team won the Non-Public B state championship in 2002, 2003 and 2012. ==Notable alumnae==