W. Leon Mason, 74, a retired principal of J.E.B. Stuart High School in Falls Church who was active in professional organizations, died of pneumonia Friday in a hospital in Lewes, Del. A former resident of Falls Church, he had lived in Dagsboro, Del., since 1971.

Mr. Mason was principal of Stuart High from 1959 until retiring in 1970. He began his career as a school administrator in Northern Virginia as assistant principal of Mount Vernon High School in Fairfax County in 1954. He was principal of Falls Church High for three years before going to Stuart.

He was president of the Virginia chapter of the National Association of Secondary School Principals from 1968 to 1970. He also was a member of the National, Virginia and Fairfax education associations and the Virginia Athletic League.

He was a past president of the Falls Church chapter of the Kiwanis.

Mr. Mason, who was born on a farm in Accomack County, Va., was a graduate of Randolph-Macon College. He was a teacher and administrator in schools in southside Virginia from 1929 to 1942. During World War II, he worked for the War Department. From 1947 to 1952, he was training director of the Army's Post Exchange Service in New York.

Survivors include his wife, Margaret S., of Dagsboro; a son, William F., of Herndon; a daughter, Elizabeth Fagan of Newport News, Va., and four grandchildren.