After studying Law at Edinburgh University (1944 - 1946) and London's Middle Temple (1948) Aitchison attended Slade School of Fine Art in London (1952 - 1954).
His work is characterised by the use of intense, pure colour to describe shape and form in extremely spare compositions. His subject matter is traditional, featuring religious themes, landscapes, portraits and still-lifes. In 1955 Aitchison was awarded the British Council Italian Government Scholarship for painting and travelled to Italy, where the clear light and natural 'Biblical' landscapes had a profound influence on his work.
Aitchison's religious scenes are not of an ecclesiastical discipline, but have a timeless, poetic and mysterious atmosphere reminiscent of 15th-century miniatures. In Crucifixion in a Landscape (1967 - 1970; S. York priv. col.) the figure of Christ is only slightly more substantial than a mirage, denoting 'spiritual essence' rather than solid substance and blurring the distinction between the real and the imaginary. In his portraits colour and composition are key; Aitchison has often shown a predilection for black models, enjoying the way colour reflects against dark skin. Model Standing Against Blue Wall (1962; London, Tate) is one of the many paintings of the 1960s and 1970s of the model Georgeous Macaulay where compositional requirements take priority over truth; the figure standing in the distance is dominated and framed by the colourful abstract shapes of the background.
In 1978 Aitchison was elected an Associate Member and in 1988 a Member of the Royal Academy of Arts, London.