Of volcanic origin, St. Helena has much volcanic ash and many conspicuous rock features, but volcanic activity on the island is now extinct. Perpendicular cliffs rise 1,600 to 2,300 feet (490 to 700 metres) on the eastern, northern, and western sides of the island. A semicircular rim of mountains north of Sandy Bay includes the island's two highest points, Diana Peak (2,690 feet 【820 metres】) and Mount Actaeon (2,625 feet 【800 metres】). South of the mountains, water-cut gorges are dispersed, becoming deep valleys near the sea. Springs are numerous. The only practicable place for ship landings is on the island's northwestern side at James Bay, from which a narrow valley extends 1.5 miles (2.4 km) inland. In this valley is nestled the town and port of Jamestown.

St. Helena lies in the path of cool South Atlantic trade winds, and its climate is temperate, with warm summers and only slightly cooler winters. Annual rainfall ranges from 8 inches (200 mm) at sea level and more than 30 inches (760 mm) at the island's centre.

St. Helena's native flora and fauna, which existed in isolation for millions of years, were devastated by the arrival of humans beginning in the 16th century, and subsequent introductions of alien species have continued to cause damage. Unique species still survive, however. The island can be divided into three vegetation zones: a rocky, barren coastal zone with cactus, extending inland for about 1 mile (1.6 km); a middle zone, extending another 0.25 to 1 mile (0.4 to 1.6 km) inland and supporting grassy hillslopes and gorse, willows, poplars, and Scotch pines; and the central zone, about 3 miles (5 km) long and 2 miles (3 km) wide, which is the home of the island's many species of endemic plants (including the St. Helena ebony, St. Helena redwood, and black cabbage trees) as well as oak trees, cedars, eucalypti, bamboo, and banana plants. Animals native to the island tend to be small in size; they include invertebrates such as blushing snails and a variety of insects, as well as wirebirds (a type of plover).

The island's population is largely of mixed European (mostly British), Asian, and African descent. English is the only language spoken, and the majority of the people are Anglicans. Jamestown, the only town on St. Helena, has about one-fourth of the island's population.

Less than one-third of the island is suitable for farming or forestry. Principal crops are corn (maize), potatoes, and green vegetables. Poultry, sheep, goats, cattle, and pigs are raised. Some coffee is grown for export. There are no minerals and virtually no industry, but some locally grown timber is used for construction purposes. Fish, primarily tuna, are caught in the waters around St. Helena; some is frozen at a cold-storage facility in the vicinity, and the remainder is dried and salted on the island. Virtually the entire output is exported.

About two-thirds of the colony's budget is provided by the United Kingdom in the form of a subsidy; the remainder is raised from the sale of postage stamps and from customs duties and wharf fees. The island began developing tourism on a small scale in the late 1990s. Unemployment is a persistent problem, and many residents work abroad, especially on Ascension Island.

The only port, Jamestown, has good anchorage for ships, and there is passenger and cargo service by sea to the United Kingdom and South Africa. The island has a system of roads but no airport or airfield.

St. Helena and its dependencies are administered by a governor, representing the British crown. The Legislative Council consists of the speaker, three ex officio members (the chief secretary, financial secretary, and attorney general of St. Helena), and 12 members who are elected to four-year terms. The governor presides over and is advised by the Executive Council, which includes the same three ex officio members who serve on the Legislative Council and five of the elected members of the Legislative Council; the latter chair the council committees and oversee departments of the government. Though the territory's residents are British subjects, legislation passed in Britain in 1981 established that they are not full citizens of the United Kingdom, as had been traditionally held. This greatly limited the population's opportunities for employment abroad, and the territory's constitutional relationship with Britain again came under review in the late 1990s. Education is compulsory and free for children between the ages of 5 and 15.

The island was discovered on May 21, 1502, which in the Eastern Orthodox Church is the feast day of St. Helena, mother of the Roman emperor Constantine. The island's discoverer, João da Nova (Nova, João da), was a Spanish navigator in the service of Portugal. The existence of the island was known only to the Portuguese until 1588, when the English navigator Capt. Thomas Cavendish (Cavendish, Thomas) visited St. Helena on his return from a voyage around the world. The island soon became a port of call for ships en route between Europe and the East Indies.

The Dutch may have occupied St. Helena about 1645–51, but in 1659 the English East India Company took possession of the island. After a brief Dutch occupation in 1673, the East India Company was confirmed in its ownership. By 1673 nearly half the inhabitants were imported slaves, but between 1826 and 1836 all slaves were freed. The remoteness of St. Helena made it attractive to the powers of Europe as a place of exile for Napoleon I, and he was confined at Longwood House on the island from October 1815 until his death in May 1821. During that period the island was placed under the jurisdiction of the British crown. Subsequently the East India Company resumed control until 1834, when the authority of the crown was restored. St. Helena remained reasonably prosperous as a busy port of call until about 1870; thereafter steam started replacing sail in seafaring, and the Suez Canal opened (1869), changing the pattern of sea routes.

In the early 1960s a telecommunications centre was developed on the island of Ascension, and employment there of workers from St. Helena restored a degree of prosperity. St. Helena was given some measure of self-rule through an Order in Council and Royal Instructions in 1966 (effective January 1967) that provided for local executive and legislative councils; this order was replaced by a new constitution that became effective in January 1989.