[3] In 2020, Premadasa split from the UNP to lead his own political party, the Samagi Jana Balawegaya, and ran in the 2020 parliamentary elections as the leader and prime ministerial candidate of the new alliance.

Premadasa won the A/L prize for politics and business studies and besides being appointed a prefect, also played in the first 11 cricket team for four years, captaining the side in 1986.

[10] Whilst completing his master's degree in the United States at the University of Maryland, College Park, he returned to Sri Lanka following his father's assassination.

He initiated the youth movement Tharuna Saviya, the People Development Foundation Jana Suwaya for poverty alleviation in Hambantota, and the Sasunata Aruna to aid Buddhist temples[12] and Sunday schools in line with the 2600 Sambuddhathwa Jayanthi celebrations.

[15] Following the 2015 presidential election, Sajith Premadasa was appointed Minister of Housing and Samurdhi in the new cabinet formed in January 2015 by President Maithripala Sirisena.

The power struggle between the president and UNP prime minister Ranil Wickremesinghe led to the 2018 Sri Lankan constitutional crisis.

[20] UNP leader Ranil Wickremesinghe was the obvious choice for the candidacy and he reportedly expressed his intentions to run for the presidency in a meeting with other senior members.

Meanwhile, several rebel MPs including Mangala Samaraweera, Harin Fernando, Ajith Perera, Harsha de Silva, Sujeewa Senasinghe wanted deputy leader Premadasa as the party's candidate and organised a rally in Badulla where Premadasa openly expressed his intentions to be the UNP candidate.

[22] The Premadasa faction gained strong support from fellow party members through successful rallies in Matara,[23] Kurunegala[24] and Matugama.

Premadasa was defeated by SLPP candidate Gotabaya Rajapaksa, who campaigned on a pro-nationalistic, economic development and national security platform.

At the same time, Premadasa gained a majority in areas dominated by Tamil and Muslim minorities, whom had been greatly affected by the country's civil war.

[28][29] With his appointment as Leader of Opposition, he was automatically given a seat as a Member of Constitutional Council (Sri Lanka).

Premadasa refused an invitation by President Gotabaya Rajapaksa to accept the premiership during the 2022 Sri Lankan political crisis.