[15] He first entered the Parliament of Sri Lanka as a representative of the Matara District in 1989, and he was appointed Minister of Post and Telecommunications in the cabinet of President Chandrika Bandaranaike Kumaratunga in 1994.

[21] On 9 February 2007, he was sacked from the cabinet together with ministers Anura Bandaranaike and Sripathi Sooriyarachchi after falling out with the then president Mahinda Rajapaksa.

[25][26] During the 2015 Presidential election, he played a pivotal role as a joint spokesperson of the opposition coalition candidate Maithripala Sirisena.

[11][27] After the 2019 Presidential election, Samaraweera joined the newly formed Samagi Jana Balawegaya which was launched in 2020 and led by opposition leader Sajith Premadasa.

[28] Mangala was reported to have played a major role in the break up of the United National Party and also led the formation of Samagi Jana Balavegaya.

[29] In June 2020, he announced that he would not contest the 2020 parliamentary election which was held on 5 August 2020 and urged the public not to vote for his preferential number, stating that it had become impossible by that time to get his name removed from the nomination list.

[40][41] In November 2018, after homophobic comments made by President Maithripala Sirisena, he wrote on Twitter that “I would rather be a butterfly than a leech Mr.

!”[42] In May 2021, he featured in a YouTube video, "Light Upali", where he played the role of a warrior defending social media platform TikTok.

[45][46][47] Prior to his death, he was admitted into the intensive care unit of the Lanka Hospital in Colombo after being tested positive for COVID-19 in August 2021.