Guthrie had initially been offered a role with British Overseas Airways Corporation immediately after the war, but the death of his father on 28 September 1945 made this difficult and he would instead manage the Guthrie family's business interests, which included the shareholding in North Central Wagon Company. He was appointed to the board of North Central in August 1946, at their annual general meeting. He initially asked Edward Heath in 1946 to join North Central as the deputy chairman, with a view to him taking over as chairman, but Heath, who was interested in running for Parliament, thought moving from London to Rotherham, where North Central was based, would be damaging to his political ambitions and turned down the offer. Guthrie, in addition to his North Central role, served as managing director of the merchant bank Brown, Shipley & Co., and he would again offer Edward Heath a role, in 1949, as a trainee banker. The role was to provide a source of income for Heath, who had already been selected as the prospective parliamentary candidate for Bexley, and who would be elected to Parliament at the 1950 General Election, held in February. Guthrie took up the deputy chairman role at North Central Wagon Company himself, serving from 1960 to 1964. He also served on the board of Radio Rentals until 1964, and served on the board of Prudential Assurance Company from May 1963 to January 1964. He was appointed to fill the vacant seat on the Prudential board created by the death of Donald Fergusson in March 1963, and it was widely expected he would become Prudential's chairman. His knowledge of aviation and finance made him an ideal candidate for board positions with the state-owned British airlines; firstly he was appointed to the board of British European Airways in April 1959 then in 1963, he was appointed to the board of British Overseas Airways Corporation, where he took over from Matthew Slattery as chairman and Basil Smallpeice as managing director, taking the additional title of chief executive. He took up his position on 1 January 1964, relinquishing all but his British European Airways board appointment when taking up the BOAC role; the Government wished for closer co-operation between BOAC and BEA, and it was agreed that Guthrie would remain on the BEA board to facilitate this. Guthrie joined the Air Registration Board (predecessor of the Civil Aviation Authority) in September 1964, serving as an airline representative. He was appointed at the same time as his predecessor at BOAC, Matthew Slattery and aircraft engine designer Stanley Hooker.