Bethune was elected to the Tasmanian House of Assembly at the 1946 general election on 23 November 1946, representing the seat of Wilmot for the newly formed Liberal Party. On 19 March 1960, Bethune became Leader of the Opposition when the leader of the Liberal Party, Tim Jackson resigned in protest at party disunity and a lack of support for his leadership. On 7 September 1966, Kevin Lyons resigned from the state Liberal Party after a dispute over preselection, and on 15 October formed the Centre Party. In 1969, Lyons held the balance of power, and offered his support to Bethune to form government with himself as Deputy Premier. Bethune accepted. This forced Labor Premier Eric Reece to resign on 26 May, making Bethune the first non-Labor Premier of Tasmania in 35 years, and the first to hold the post since the main non-Labor party in Tasmania adopted the Liberal banner in 1946. He also served as his own Treasurer. During his term as Premier, Bethune introduced a number of important initiatives, although he had pledged to avoid "revolutionary changes". Bethune's government introduced random breath tests to tackle drink driving, made the wearing of seatbelts compulsory, set up the Tasmania Parks and Wildlife Service, built more schools and tightened state finances. Bethune was instrumental in the establishment of a Hansard service for the Tasmanian parliament, although he was no longer Premier by the time the system was introduced. Kevin Lyons had lost a vote to introduce Hansard in 1960, and with the Bethune-Lyons coalition in power in 1969, Bethune sought a report into the feasibility of such a transcription service. The report was not completed until 1973, but Hansard was considered by Bethune's successors Eric Reece and Doug Lowe until finally being implemented in 1979. In 1972, Kevin Lyons dissolved the coalition between the Centre and Liberal parties, and was highly critical of Bethune. Bethune was forced to the polls, and the instability in his government saw him lose power in the election on a three-seat swing. Labor won a clear majority, and Reece returned as premier.
After politics
Bethune retired from politics on 30 June 1975, and later joined forces with his former rival, Eric Reece, to back the Franklin Dam which they both supported. Sir Angus died after a short illness on 22 August 2004 in Hobart, aged 95. He was accorded a state funeral which was held on 27 August.
