In 1815 the United Kingdom formally annexed the islands, ruling them from the Cape Colony in South Africa. This is reported to have primarily been a measure to ensure that the French would not be able to use the islands as a base for a rescue operation to free Napoleon Bonaparte from his prison on Saint Helena. The occupation also prevented the United States from using Tristan as a base, as they had during the War of 1812. Attempts to colonise Inaccessible Island failed.

The islands were occupied by a British military garrison, and a civilian population was gradually built up. Whalers also set up on the islands as a base for operations in the Southern Atlantic. However the opening of the Suez Canal in 1869, together with the move from sailing ships to coal fired steam ships, saw the increased isolation of the islands, as they were no longer needed as a stopping port for journeys from Europe to the Far East.