Sister Mary Stephen Cassidy, R.S.M., 99, a member of the Brooklyn Sisters of Mercy for 80 years, died March 19 at Maria Regina Residence, Brentwood.

Brooklyn born, she attended St. Thomas Aquinas School, Flatlands, and Mercy Juniorate H.S., Fort Greene.

She entered the Sisters of Mercy Novitiate in Syosset in 1938 and professed her final vows in 1943.

In 1955, she received a bachelor’s degree in education from Manhattan College followed by a master’s in social studies from St. John’s University in 1965. She received a certificate in Chinese Culture from the NY Chinese Institute in 1969 and a certificate in International Politics of Southeast Asia from the NY Asian Institute.

She taught at St. Patrick, Kent Ave., 1940-41; St. Brigid, Bushwick, 1941-43; Holy Rosary, Bedford-Stuyvesant, 1943-46, 53-55; Our Lady of Mercy Academy, Syosset, 1955-57, 60-61; Holy Innocents, Flatbush, 1947-50; St. Therese of Lisieux, East Flatbush, 1950-53; Mercy H.S., Riverhead, L.I., 1957-60; Mater Christi H.S., 1961-77; St. Brendan’s H.S., 1977-78; and Stella Maris H.S., Rockaway Beach, 1978-93.

She also served one year at the Convent of Mercy, 1946-47.

She was predeceased by nine brothers and sisters.

A Mass of Christian Burial was celebrated in the Sisters of Mercy Convent, Whitestone. Burial was in Holy Cross Cemetery, Brooklyn.