The Rev. Richard Post Williams, 78, an Episcopal priest who was a canon at the Washington Cathedral, a special assistant at the Department of Health, Education and Welfare and a deputy director of the Federal Council on Aging, died of pneumonia June 8 at the Avalon Nursing Home in Hagerstown, Md. He had a stroke.

Mr. Williams came to Washington in 1946 after World War II service as a Navy chaplain in Europe. He was associate rector of the Church of the Epiphany until 1949, when he was assigned to the cathedral.

He was director of the department of social relations for the Washington diocese of the Episcopal Church until 1965.

He then became vicar to the bishop and head of the Holy Cross Episcopal Mission. He also was a consultant to various social programs.

From 1970 to 1972, he was a special assistant at HEW, the predecessor of the Department of Health and Human Services. From 1973 until he retired in 1978, he was deputy director of the Federal Council on the Aging.

Mr. Williams was born in Knoxville, Tenn. He graduated from the University of Tennessee and received a doctorate in divinity at Princeton University.

He also studied at Columbia University and the Washington School of Divinity. He lived in Chevy Chase until moving to the nursing home in Hagerstown about 1982.

Mr. Williams was a member of Cabin John Volunteer Fire Department, the Bethesda Country Club, the Fairfax Hunt, the National Steeplechase and Hunt Association, the National Press Club, the City Tavern Club and the Sons of the Revolution.

Survivors include his wife, the former Anita Sutton, whom he married in 1960, of Chevy Chase, and two daughters, Casey Williams of Washington and Lesley Williams of New York City.