Donald M. Merritt, 76, a retired commissioner of the public debt who served more than 35 years with the Treasury Department, died of cancer Sunday at Sibley Memorial Hospital.

Named commissioner in 1960, he was responsible for administration of the public debt, which was almost $400 billion by the time he retired in 1971. Mr. Merritt received the Treasury Department's Exceptional Service Award when he left government.

He was born in Iowa City, Iowa, and graduated from the University of Iowa. He worked for securities and accounting firms in Los Angeles until 1933 when he became assistant state auditor for the California Relief Association.

He began his federal career in 1935 in the San Francisco office of the Treasury Department. In 1941, he transferred to Treasury's bureau of accounts here. In 1954, he was named assistant commissioner in 1946, and returned to Washington.

Mr. Merritt was a member of the American Accounting Association, the American Political Science Association and the American Society for Public Administration. He also belonged to the University Club, Saints and Sinners, and the Newcomers Club.

He was a member of the St. Margaret's Episcopal Church.

Survivors include his wife, Doris, of the home in Westmoreland Hills in Bethsda, and a sister, Mrs, Ralph L. Byrnes, of Los Angeles.