After the war Downer joined the Liberal Party of Australia and in 1949 he was elected to the House of Representatives for a rural South Australian seat. He was Minister for Immigration from 1958 to 1963. He retired from Parliament on his appointment as Australian High Commissioner in London, a position he held until 1972. He was knighted in 1965, becoming Sir Alexander Downer. He was made a Freeman of the City of London in 1965.
Sir Alec's son, Alexander Downer, is also a Liberal politician, who was Leader of the Opposition 1994-95 and has been Foreign Minister of Australia since 1996.