Aitken learned chess from his father at age 10. He was Scottish champion in 1935, 1952, 1953, 1955, 1956, 1957, 1958, 1960, 1961 and 1965, the latter jointly with PM Jamieson. He was also London Champion in 1950. In 1959 he had his best result in the British Championship, finishing tied for seventh place. Aitken represented Scotland in four Chess Olympiads. He played top board at Stockholm 1937, scoring only 32.4% but he did defeat Swedish GM Gideon Ståhlberg and draw with American GM Samuel Reshevsky. He played second board at Munich 1958 and Tel Aviv 1964, scoring 67.6% and 28.1% respectively. Aitken played sixth board at Skopje 1972, scoring 38.9%.