Sister Margaret Casey, R.S.M., 79, formerly Sister Mary Xavier, a Sister of Mercy for 61 years, died at Maria Regina Residence in Brentwood, L.I., Aug. 13.

Born in Brooklyn, she attended St. Agatha School, Sunset Park, and Mercy Juniorate H.S., Fort Greene.

She entered the Sisters of Mercy Novitiate in Syosset, L.I., on Sept. 8, 1954, and professed her final vows on March 23, 1960.

She earned a degree in French from St. Francis College, Brooklyn Heights, and a professional diploma in counselor education from Long Island University.

She taught at Sacred Heart, Adelphi St., 1957-59; St. Thomas Aquinas, Flatlands, 1959-66; and St. Brigid, Bushwick, 1966-68.

She also taught at Holy Trinity H.S., Hicksville, L.I., 1969-75, where she also served as guidance counselor, 1975-2015.

She is survived by her sister, Elizabeth Gambino.

A Mass of Christian Burial was held at Our Lady of Mercy Academy, Syosset, L.I., Aug. 17. Burial was in St. Charles Cemetery, Farmingdale, L.I.