Aitchison was born in Edinburgh, the son of the lawyer, politician and judge Craigie Mason Aitchison. His grandfather, Reverend James Aitchison, was minister at the United Free Church Erskine Kirk in Falkirk. Aitchison was educated at Loretto School, Musselburgh, East Lothian until the death of his father in 1941 and then at home by private tutors. His mother, Lady Aitchison, played international hockey. Her family owned Tulliallan, an estate in Fife, where Aitchison did some of his first landscape painting. He was rejected for military service in the Second World War on medical grounds. He studied law at Edinburgh University from 1944 to 1946, and at the Middle Temple in London in 1948, before changing career. He returned to Edinburgh in 1950 to practise painting in a converted mews house in Church Lane, and then studied at the Slade School of Fine Art in London from 1952 to 1954 under William Coldstream and Robert Medley. Aitchison won a prize for the best still life his second year. Fellow students included Michael Andrews, Tony Pacitti, Philip Sutton (artist), Victor Willing, Paula Rego, Myles Murphy and Euan Uglow. Aitchison remained friends with Uglow, and was best man at his wedding. Aitchison was awarded a British Council scholarship in 1955 to study in Italy. He toured the country, and was influenced by early Italian painting, particularly Piero della Francesca.[5] He returned to Scotland, but moved to Kennington in London in 1963.