UNMIL superseded the United Nations Observer Mission in Liberia (UNOMIL), which had been established in 1993 to support the peacekeeping efforts of the Economic Community of West African States (ECOWAS) during the First Liberian Civil War (1989–1996).

The mission would be regularly extended for another fifteen years, during which it successfully facilitated two free and fair elections, maintained security, and helped rebuild infrastructure and political institutions, often in close cooperation with local civil society groups.

[4] By June 2016, UNMIL's mandate was officially transferred to local authorities, with the force reduced to 1,240 U.N. military and 606 police personnel only in case of emergency;[5] the peacekeeping mission formally withdrew on 30 March 2018.

From the outset of the conflict, a sub regional organization, the Economic Community of West African States (ECOWAS), undertook various initiatives aimed at a peaceful settlement.

The Security Council in 1992 imposed an arms embargo on Liberia, and the Secretary-General appointed a Special Representative to assist in talks between ECOWAS and the warring parties.

That first United Nations post-conflict peace-building support office was tasked primarily with assisting the Government in consolidating peace following the July 1997 multiparty elections.

Meanwhile, the promotion of national reconciliation was undermined by systematic abuses of human rights, the exclusion and harassment of political opponents and the absence of security sector reform.

On 8 July 2003, as fighting between Government forces and various warring factions intensified and humanitarian tragedy threatened, the Secretary-General decided (S/2003/695) to appoint Jacques Paul Klein of the United States his Special Representative for Liberia.

As requested by the Security Council, the Secretary-General submitted on 11 September a report (S/2003/875) providing an update on the situation in the country, and containing his recommendations on the role the United Nations could play to facilitate the effective implementation of the Comprehensive Peace Agreement, as well as on the size, structure and mandate of a peacekeeping operation in Liberia.

The Special Representative would be assisted by a senior management team consisting of, among others, two Deputies, a Force Commander with the rank of Lieutenant General, and a Police Commissioner.

An HIV/AIDS policy adviser, with supporting staff, would also be attached to the Office of the Special Representative, to coordinate activities in the Mission area for the prevention of HIV transmission among civilian and military personnel and host communities.

In September 2003, the Security Council welcomed the Secretary-General's report of 11 September 2003 and its recommendations[11] and unanimously adopted Resolution 1509 establishing UNMIL with up to 15,000 United Nations military personnel, including up to 250 military observers and 160 staff officers, and up to 1,115 civilian police officers, including formed units to assist in the maintenance of law and order throughout Liberia, and the appropriate civilian component.

Among other things, the Council also took note of the intention of the Secretary-General to terminate the mandate of UNOL and to transfer the major functions performed by that Office to UNMIL.

In a statement issued on that day, the Secretary-General welcomed this very important development and saluted ECOWAS for its role in establishing the security climate that paved the way for the deployment of UNMIL.

He commended the Governments of Benin, Gambia, Ghana, Guinea-Bissau, Mali, Nigeria, Senegal and Togo who have contributed to UNMIL, as well as the United States for its support to the regional force.

[13][14] In 2018, through the Deputy Secretary General for the Rule of Law, Waldemar Wray's office, the UNMIL has "provided textbooks, furniture, and equipment" to establish a human right library at the Ashmun Street Headquarter of the Liberia National Bar Association (LNBA).

Wray stated that the library "is meant to primarily enhance the human right's knowledge of members of the bar" giving the lawyers the needed resources to aid in their ability to "protect the interests of their clients.