Thomas V. Leahy, 69, a retired engineering official with the Agency for International Development, died Friday at Georgetown University Hospital. He had a heart ailment.

He was chief of the water and sewer branch in the engineering division of AID from 1961, when the agency was established, until his retirement in 1968. Before that he had served for seven years in Tehran, Iran, as adviser and contracts officer with the old Foreign Operations Administration and then as industrial specialist and supervisory civil engineer with the old International Cooperation Administration.

Mr. Leahy was born in New York City. He earned a degree in civil engineering from Manhattan College and later attended the University of Maryland. He was a civilian engineer with the Fort Supply Dam in Oklahoma until entering the Army in 1943. He served in the China-Burma-India theater during World War II.

After the war, he worked for an engineering firm in New York City. He joined Sawyers Engineers here in 1948, remaining with that firm until 1954. He lived in Wheaton.

Mr. Leahy was a member of the National Association of Government Engineers and the Water Pollution Control Federation. He belonged to the John Carroll Division of the Hibernians, the Knights of Columbus and the Holy Name Society of St. Catherine Laboure Catholic Church.

He is survived by his wife, the former Margaret Ryder, of Wheaton; a son, Bernard R., of Bloomfield Hills, Mich.; a daughter, Etta Susanne Haggerty of Crofton, Md.; two sisters, Molly Hanlan of Port Chester, N.Y., and Kitty Mohlsick of North Highlands, Calif., and six grandchildren.