The Colonial Service, also known as His/Her Majesty's Colonial Service and replaced in 1954 by Her Majesty's Overseas Civil Service (HMOCS), was the British government service that administered most of Britain's overseas possessions, under the authority of the Secretary of State for the Colonies and the Colonial Office in London. It did not operate in British India, where the same function was delivered by the Indian Civil Service (ICS), nor in the Anglo-Egyptian Sudan, which was administered by the Sudan Political Service (SPS), nor in the internally self-governing colony of Southern Rhodesia, which had its own civil service.

The British Government's overall responsibility for the management of the territories overseas in the early 19th century lay with successive departments dealing with the various colonies and "plantations", until in 1854 a separate Colonial Office was created headed by a Secretary of State for the Colonies. That office was not responsible for the territories of the Indian Empire, including Burma, nor for Sudan which was linked to Egypt, nor for Southern Rhodesia which came under the Dominions Office. In 1966 the Colonial Office was merged with the Commonwealth Relations Office (CRO) to form the Commonwealth Office. In 1968 that Office was merged with the Foreign Office, renamed as the Foreign and Commonwealth Office (FCO), which maintained ultimate oversight of the dwindling residual HMOCS personnel until 1997.

The historical record of the Colonial Service begins with the publication on 30 March 1837, in the reign of King William IV, of the first set of what became known as Colonial Regulations, relating to "His Majesty's Colonial Service". It can therefore be said to have been the oldest of all the overseas services, predating the formation of the Indian Civil Service in 1858 and the Sudan Political Service in 1899, and in the course of its existence controlled 42 territories.

Initially there was no actual corps of officers employed in the colonies and "plantations", but that changed as the colonial empire grew during the rest of that century and into the 20th century. During the 1920s there were progressive moves towards the unification of the varied types of service that developed across the widely differing territories. In 1931 a unified Colonial Service was created, initially with sixteen sub-services, with four more added after the Second World War. Recruitment included candidates from the four self-governing Dominions: Canada, Australia, New Zealand and South Africa and a few of the colonies themselves, such as Southern Rhodesia, Malta, Cyprus, and some in the Caribbean. The officers were members of the civil service of the particular territory in which they served, as well as belonging to the relevant sub-service of the Colonial Service.

Finally, in 1954, they were all combined under the title of Her Majesty's Overseas Civil Service (HMOCS). That continued until 1 July 1997, when the largest remaining colony, Hong Kong, was handed over to the People’s Republic of China, designated as a Special Administrative Region. Thereafter the relatively few posts in the small remaining colonies (now British Overseas Territories) were filled by ad hoc British government appointments and overseen by two Ministers of the British government.

The formal end of HMOCS in 1997 was marked by a special commemorative service held in Westminster Abbey on 25 May 1999, attended by Queen Elizabeth II and Prince Philip, Duke of Edinburgh.^{[citation needed]}

The physical memorial "To all who served the Crown in the Colonial Territories" is displayed in the South Cloister of Westminster Abbey, unveiled by H.M. The Queen in March 1966.

During the 1930s the following sixteen sub-services were brought into the unified Colonial Service: