Mary Jeanne O'Reilly, 54, a member of the board of trustees of the Fairfax County Public Library and a resident of the Washington area since 1946, died Friday at Fairfax Hospital. She had cancer.

Mrs. O'Reilly was born in St. Louis.She attended Webster College there and in 1946 married Thomas J. O'Reilly. The couple moved to Washington in the same year.

In 1973, Mrs. O'Reilly earned a bachelor's degree in English from George Mason University. She was a staff assistant at WNVT (Channel 53), the public television station in Northern Virginia, in 1974 and 1975. In 1976 and 1977, she was director of transportation at the Arlington Red Cross and helped establish a shopping program for handicapped and elderly persons.

She joined the board of the Fairfax Public Library in 1976 and remained on it until her death.

Mrs. O'Reilly also had been a member of the Mason District Committee of the Democratic Party in Fairfax and a member of the Army-Navy Country Club in Arlington.

In addition to her husband, of Falls Church, where the family lives, survivors include two daughters, Nancy J. Wuerker of Reston, and Margaret A. O'Reilly of Boston; five sons, Thomas J. Jr. of Boston, John P., James M., and William V., all of Charlottesville, and Michael L., of Arlington, and three grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to Arlington County Chapter of the American Red Cross, or to the American Cancer Society.