Margaret Conlyn Nixon, 79, a retired employe of the Department of Agriculture and a longtime resident of the Washington area, died Saturday at the Fairfax Nursing Home after a stroke.

Mrs. Nixon was a native of Washington and a 1927 graduate of George Washington University's law school. She was an administrator with the Agriculture Department in the late 1940s and early 1950s.

She attended St. John's Episcopal Church in Georgetown. During the 1950s, she did volunteer work with the National Council of Churches.

Mrs. Nixon had lived in Madison, Wis., from 1937 until returning to Washington in the early 1940s. She made her home in Brentwood, Md., for about five years before moving to Arlington in the mid-1950s. In 1971, she moved to Falls Church, where she lived until moving to the nursing home in August 1979.

Her husband, Robert Arleigh Nixon, an attorney with the World War II Office of Price Administration, died in 1948. Their son, John Arleigh Nixon, a Marine Corps lance corporal, was killed in action in Vietnam in 1968.

Mrs. Nixon's survivors including two daughters, Pearson N. Biddle of Vienna, and Helen N. Harrell of Falls Church; four brothers, Pearson C. Conlyn of Alexandria, William J. Conlyn of Kilmarnock, Va., and Robert M. Conlyn of Prince Frederick, Md., and four grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the American Cancer Society, or to the Vietnam War Memorial Fund.