From 1908, entrants to the Colonial Administrative Service destined for West and East Africa were offered training on a short course at the Imperial Institute in London. In 1926, as part of the expansion of the Colonial Service following the end of the First World War, Sir Ralph Furse persuaded the Colonial Office to transfer the training course to the universities at Oxford and Cambridge. The course was originally called the Tropical African Service (TAS) course, although it was renamed the Colonial Administrative Service (CAS) course in 1933, reflecting its opening to Eastern cadets too. Initially the course lasted for two terms (four months) which enabled a probationer to embark on their training in either October or January, but this ultimately changed so that the training period coincided with the full academic year. The training at the universities moved away from the practical subjects taught at the Imperial Institute and became more academic, aiming to provide a theoretical introduction to colonial government. An African language, anthropology, law, agriculture, forestry and 'British Rule in Africa' now dominated the syllabus, under the direction of the colonial historians Professor Sir Reginald Coupland (1884-1952) at Oxford and Professor Eric Walker (1886-1976) at Cambridge. The services of the Imperial Institute were retained for the teaching of tropical medicine and accounting during the Oxford and Cambridge vacations (six weeks at Christmas and Easter).

As part of the post-war reorganisation of the Colonial Service, the Colonial Office set up the Devonshire Committee to make recommendations for post-war training. The Devonshire Committee sat throughout 1944 and 1945 and, in their report, recommended that there be two courses. The first, called the Devonshire 'A' Course - or First (Devonshire) course - would be a full years course scheduled to start in October 1947 at Oxford, Cambridge and also at the London School of Economics (LSE). There was also to be a Devonshire 'B' Course - or Second (Devonshire) course - which would be a sabbatical year, also offered by all 3 institutions. The Devonshire 'B' Course proved especially valuable for those who, having had their university education interrupted (or perhaps cancelled) by war service, had joined the CAS in 1945 or 1946 and been sent straight out to the colonies without the opportunity to attend the Devonshire 'A' Course. The Committee also argued that a Colonial Service club in each university would be essential as a meeting place for all those taking the course. The Devonshire courses (which were renamed Overseas Service courses in 1954) continued until 1969.