He did not seek re-election to the House in the 1949 election. He was appointed to the Senate of Canada representing the senatorial division of Edmonton, Alberta. From 1949 to 1950, while a senator, he was a minister without portfolio in the cabinet of Louis St-Laurent.
MacKinnon married Irene Sharpe from Prince Edward Island on June 28, 1911, and had one adopted daughter, Keltie. She died on October 31, 1968, leaving her husband Denis Slattery, and five children, Sandy White (née Slattery), Gayle Slattery, John Slattery, Jim Slattery and Ann Varszegi (née Slattery).
1The office of Minister of Marine and Fisheries was abolished and the offices of Minister of Fisheries and Minister of Marine were created in 1930. The Minister of Marine office was a precursor to the Minister of Transport. 2From 1971 to 1976 the Minister of the Environment was also the Minister of Fisheries.