The Temne were living along the northern coast of present-day Sierra Leone when the first Portuguese navigators reached the region in 1460. The Portuguese landed on the Sierra Leone Peninsula, naming it Serra Lyoa [lion mountains] after the mountains located there. Beginning c.1500, European traders stopped regularly on the peninsula, exchanging cloth and metal goods for ivory, timber, and small numbers of slaves. Beginning in the mid-16th cent. Mande-speaking people migrated into Sierra Leone from present-day Liberia, and they eventually established the states of Bullom, Loko, Boure, and Sherbro. In the early 17th cent. British traders became increasingly active along the Sierra Leone coast. In the early 18th cent. Fulani and Mande-speaking persons from the Fouta Djallon region of present-day Guinea converted numerous Temne of N Sierra Leone to Islam. Sierra Leone was a minor source of slaves for the transatlantic slave trade during the 17th and 18th cent.

Following the American Revolutionary War (1775-83) attempts were made to resettle freed slaves who had sided with Great Britain in Africa. In 1787, 400 persons (including 330 blacks and 70 white prostitutes) arrived at the Sierra Leone Peninsula, bought land from local Temne leaders, and established the Province of Freedom near present-day Freetown. The settlement did not fare well, and most of the inhabitants died of disease in the first year. A renewed attempt at settlement was made in 1792, when about 1,100 freed slaves under the leadership of the abolitionist Thomas Clarkson landed on the peninsula and founded Freetown. They were joined by about 500 free blacks from Jamaica in 1800. The new colony was controlled by the Sierra Leone Company, which forcefully held off the Temne while the settlers supported themselves by farming.

In 1807, Great Britain outlawed the slave trade, and in early 1808 the British government took over Freetown from the financially troubled company, using it as a naval base for antislavery patrols. Between 1808 and 1864 approximately 50,000 liberated slaves settled at Freetown. Protestant missionaries were active there, and in 1827 they founded Fourah Bay College (now part of the Univ. of Sierra Leone), where Africans were educated. Most of the freedmen and their descendants, known as Creoles or Krios, were Christians. They became active as missionaries, traders, and civil servants along the Sierra Leone coast and on Sherbro Island as well as in other regions of coastal W Africa, especially among the Yoruba of present-day SW Nigeria.

During the periods 1821 to 1827, 1843 to 1850, and 1866 to 1874, British holdings on the Gold Coast (present-day Ghana) were placed under the governor of Sierra Leone. In 1863 an advisory legislative council was established in Sierra Leone. The British were reluctant to assume added responsibility by increasing the size of the colony, but in 1896 the interior was proclaimed a British protectorate, mainly in order to forestall French ambitions in the region, and the Colony and Protectorate of Sierra Leone was established.

The protectorate was ruled "indirectly" (i.e., through the rulers of the numerous small states, rather than by creating an entirely new administrative structure) and a hut tax was imposed in 1898 to pay for administrative costs. The Africans protested the tax in a war (1898) led in the north by Bai Bureh and in the south by the Poro secret society; the British quickly emerged victorious and there were no further major armed protests. Under the British, little economic development was undertaken in the protectorate until the 1950s, although a railroad was built and the production for export of palm products and peanuts was encouraged.

After World War II, Africans were given more political responsibility, and educational opportunities were enlarged. In the economic sphere, mining (especially of diamonds and iron ore) increased greatly. The Creoles of the colony, who had been largely excluded from higher government posts in favor of the British, sought a larger voice in the affairs of Sierra Leone. A constitution adopted in 1951 gave additional power to Africans. However, the Creoles were a small minority in the combined colony and protectorate, and in the elections of 1951 the protectorate-based Sierra Leone Peoples party (SLPP), led by Dr. Milton Margai (a Mende), emerged victorious.

On Apr. 27, 1961, Sierra Leone became independent, with Margai as prime minister. He died in 1964 and was succeeded by his brother, Albert M. Margai. Following the 1967 general elections, Siaka Stevens of the All Peoples Congress party (APC), a Temne-based party, was appointed prime minister by the governor-general (a Sierra Leonian who represented the British monarch). However, a military coup led by Brig. David Lansana in support of Margai ousted Stevens a few minutes after he took the oath of office.

The Lansana government itself was soon toppled and replaced by a National Reformation Council (NRC) headed by Col. Andrew Juxom-Smith. In 1968, an army revolt overthrew the NRC and returned the nation to parliamentary government, with Stevens as prime minister. The following years were marked by considerable unrest, caused by ethnic and army disaffection with the central government. After an attempted coup in 1971, parliament declared Sierra Leone to be a republic, with Stevens as president. Guinean troops requested by Stevens to support his government were in the country from 1971 to 1973. Stevens's APC swept the 1973 parliamentary elections, creating a de facto one-party state; a 1978 referendum made the APC the only legal party. Maj. Gen. Joseph Saidu Momoh succeeded Stevens as president in 1986.

In 1991 a referendum was passed, providing for a new constitution and multiparty democracy. However, in 1992, Momoh was overthrown in a military coup. Capt. Valentine Strasser soon became president, but he was ousted in Jan., 1996, and replaced by Brig. Gen. Julius Maada Bio. Promises of a return to civilian rule were fulfilled by Bio, who handed power over to Ahmad Tejan Kabbah, of the Sierra Leone People's party, after the conclusion of elections in early 1996. Kabbah's government reached a cease-fire in the war with the Revolutionary United Front (RUF), which had launched its first attacks in 1991; rebel terror attacks continued, however, aided by Liberia.

Kabbah was overthrown in May, 1997, by the Armed Forces Revolutionary Council (AFRC), a military junta headed by Lt. Col. Johnny Paul Koroma. The junta soon invited the RUF to participate in a new government. The United Nations imposed sanctions against the military government in Oct., 1997, and the Economic Community of West African States (ECOWAS) sent in forces led by Nigeria. The rebels were subdued in Feb., 1998, and President Kabbah was returned to office in March.

Fighting continued, however, in many parts of the country, with reports of widespread atrocities. Over 6,000 people were killed in fighting in the Freetown area in Jan., 1999, alone. In March, Nigeria announced it would withdraw its forces by May. A peace accord was signed in July between President Kabbah and Foday Sankoh of the RUF. The agreement granted the rebels seats in a new government and all forces a general amnesty from prosecution. The government had largely ceased functioning effectively, however, and at least half of its territory remained under rebel control.

In October, the United Nations agreed to send peacekeepers to help restore order and disarm the rebels. The first of the 6,000-member force began arriving in December, and the Security Council voted in Feb., 2000, to increase the UN force to 11,000 (and subsequently to 13,000). In May, when nearly all Nigerian forces had left and UN forces were attempting to disarm the RUF in E Sierra Leone, Sankoh's forces clashed with the UN troops, and some 500 peacekeepers were taken hostage as the peace accord effectively collapsed.

An 800-member British force entered the country to secure W Freetown and evacuate Europeans; some also acted in support of the forces (including Koroma's AFRC group) fighting the RUF. After Sankoh was captured in Freetown, the hostages were gradually released by the RUF, but clashes between the UN forces and the RUF continued, and in July the West Side Boys (part of the AFRC) clashed with the peacekeepers. In the same month the UN Security Council placed a ban on the sale of rough diamonds from Sierra Leone in an attempt to undermine the funding of the RUF. In late August, Issa Sesay became head of the RUF; also, British troops training the Sierra Leone army were taken hostage by the West Side Boys, but were freed by a British raid in September.

General elections scheduled for early 2001 were postponed in Feb., 2001, due to the insecurity caused by the civil war. In May, 2001, sanctions were imposed on Liberia because of its support for the rebels, and UN peacekeepers began to make headway in disarming the various factions. Although disarmament of rebel and progovernment militias proceeded slowly and fighting continued to occur, by Jan., 2002, most of the estimated 45,000 fighters had surrendered their weapons. In a ceremony that month, government and rebel leaders declared the civil war to have ended; an estimated 50,000 persons died in the conflict. Subsequently, a tribunal established (2002-9) by Sierra Leone and the United Nations tried and convicted Issa Sesay and two other surviving leaders of the RUF of war crimes.

Elections were finally held in May, 2002. President Kabbah was reelected, and his Sierra Leone People's party won a majority of the parliamentary seats. In June, 2003, the UN ban on the sale of Sierra Leone diamonds expired and was not renewed. The UN disarmament and rehabilitation program for Sierra Leone's fighters was completed in Feb., 2004, by which time more 70,000 former combatants had been helped.

UN forces returned primary responsibility for security in the area around the capital to Sierra Leone's police and armed forces in Sept., 2004; it was the last part of the country to be turned over. Some UN peacekeepers remained to assist the Sierra Leone government until the end of 2005. Parliamentary elections in Aug., 2007, gave a majority of the seats to opposition All People's Congress (APC), and after a runoff, Ernest Bai Koroma, of the APC, was elected president. The UN Security Council lifted its remaining sanctions on the country, including the arms embargo, in Sept., 2010. Koroma was reelected in Nov., 2012, and the APC again won a legislative majority.