The party, then called the Committee for the Liberation of São Tomé and Príncipe, was founded in 1960 as a nationalist group opposed to Portuguese colonial rule.

Manuel Pinto da Costa, who would eventually become President of an independent São Tomé and Príncipe, was the leader of the party.

Independence was achieved on 12 July 1975, with Manuel Pinto da Costa as president and Miguel Trovoada as prime minister.

The constitution promulgated on 12 December 1975, effectively vested absolute power in the President and the MLSTP became the nation's sole legal political party.

At the MLSTP Party Congress in October 1990, Carlos Graça was appointed as the new Secretary-General, in succession to Manuel Pinto da Costa.

[3] The ADI denounced Menezes' designation of the MLSTP–PSD to form a government as unconstitutional, arguing that it was too late in the parliamentary term to do so, and it took the matter to the Supreme Tribunal of Justice.

In the 1994 legislative elections, the party received 37% of the vote and regained control of the National Assembly winning 27 of the 55-seats, one short of an absolute majority.

March 1995 elections to the newly created seven member assembly on the smaller island of Príncipe resulted in another victory for the party.

In the July 2001 presidential election, Manuel Pinto da Costa again attempted to regain the presidency, but was soundly defeated by businessman Fradique de Menezes 55.2% to 40.0%.