Under the provisions of the 1833 India Act, control of Saint Helena passed from the East India Company to the British Crown, and it became a crown colony. Subsequent administrative cost-cutting triggered a long-term population decline: those who could afford to do so tended to leave the island for better opportunities elsewhere. The latter half of the 19th century saw the advent of steamships not reliant on trade winds, as well as the diversion of Far East trade away from the traditional South Atlantic shipping lanes to a route via the Red Sea (which, prior to the building of the Suez Canal, involved a short overland section). So the number of ships calling at the island fell from 1,100 in 1855 to only 288 in 1889.
In 1840, a British naval station established to suppress the African slave trade was based on the island, and between 1840 and 1849 over 15,000 freed slaves, known as "Liberated Africans", were landed there.
In 1858, the French emperor Napoleon III purchased, in the name of the French government, Longwood House and the lands around it, the last residence of Napoleon I (who died there in 1821). It is still French property, administered by a French representative and under the authority of the French Ministry of Foreign Affairs.
On 11 April 1898 American Joshua Slocum, on his famous and epic solo round-the-world voyage, arrived at Jamestown. He departed on 20 April 1898 for the final leg of his circumnavigation, having been extended hospitality by the governor, His Excellency Sir R A Standale. He presented two lectures on his voyage and was invited to Longwood by the French Consular agent.
In 1900 and 1901, over 6,000 Boer prisoners were held on the island, notably Piet Cronjé and his wife after their defeat at Battle of Paardeberg. The resulting population reached an all-time high of 9,850 in 1901.
A local industry manufacturing fibre from New Zealand flax was successfully re-established in 1907 and generated considerable income during the First World War. Ascension Island was made a dependency of Saint Helena in 1922, and Tristan da Cunha followed in 1938. During the Second World War, the United States built Wideawake airport on Ascension in 1942, but no military use was made of Saint Helena except maintenance of its defences.
During this period, the island enjoyed increased revenues from the sale of flax, with prices peaking in 1951. However, the industry declined because of transport costs and competition from synthetic fibres. The decision by the British Post Office to use synthetic fibres for its mailbags was a further blow, contributing to the closure of the island's flax mills in 1965.
From 1958, the Union Castle shipping line gradually reduced its service calls to the island. Curnow Shipping, based in Avonmouth, replaced the Union-Castle Line mailship service in 1977, using the RMS (Royal Mail Ship) St Helena which was introduced in 1989.