Donahoe ran unsuccessfully for the House of Commons of Canada as the National Government candidate in the riding of Halifax in 1940 federal election.
He was elected to the Nova Scotia House of Assembly in a 1954 by-election held after the death of Angus L. Macdonald and was re-elected four times in 1956, 1960, 1963 and 1967.
He was the Attorney General and Minister of Public Health in the cabinets of Robert Stanfield and George Isaac Smith.
In 1979, he was appointed to the Senate of Canada representing the senatorial division of Halifax, Nova Scotia.
In 1969, Pope Paul VI made him a Knight of the Order of St. Gregory the Great.