Thomas John Canning R., 78, a former mayor of Greenbelt and a retired attorney with the federal government, died Saturday at Prince George's General Hospital after a heart attack.

Mr. Canning was a member of Greenbelt's city council from 1947 to 1963. He served as mayor from 1947 to 1949 and from 1955 to 1959. He had lived in College Park since 1963.

He was an attorney with the Army Corps of Engineers for 25 years before retiring in 1966. He then worked for seven years as a negotiator with the real estate division of the Prince George's County Public Works Department before retiring a second time in 1973.

He was a member of the Mother Seton Council of the Knights of Columbus, the St. Vincent de Paul Society and the Holy Name Society.

Mr. Canning, who was born in Haverhill, Mass., lived in Greenbelt from 1942 to 1963. He came to Washington to attend Georgetown University and graduated from its law school in 1932.

Survivors include his wife, Elizabeth F., of College Park; six sons, Michael F., of Bowie, Robert W., of Dunkirk, Md., Gerard J., of Springfield, Richard ., of Beltsville, and Thomas J. Jr. and James P., both of Greenbelt; two daughters, Ann M. Mitten of Mechanicsville, Md., and Mary K. Murray of Severna Park; a sister, Gertrude Holmes of Concord, N.H., and 18 grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the Catholic Relief Service, 1011 1st Ave., New York City, 10022.