After he was awarded his bachelor's degree in science at the University of Kelaniya in 1995, he was elected the national organiser of the Socialist Students Union in 1997.

He was elected to parliament in 2004 from the Kurunegala District with the highest number of preferential votes and served as minister of Agriculture, Livestock, Land and Irrigation from 2004 to 2005.

After gaining admission to the University of Peradeniya, Dissanayake moved to Kandy and spent most of his time engaged in the clandestine political activity of the JVP.

Adopting the nom de guerre Aravinda, he undertook revolutionary work for the JVP and its military wing, the Deshapremi Janatha Vyaparaya (DJV).

[17][19] After the counterinsurgency against the JVP subsided and the situation became safe, Dissanayake came out of hiding and transferred to the University of Kelaniya in 1992, where he completed his studies and graduated in 1995 with a Bachelor of Science degree.

Dissanayake was elected to parliament from the Kurunegala District from the UPFA and was appointed by President Kumaratunga as Minister of Agriculture, Livestock, Land and Irrigation in the joint SLFP–JVP cabinet in February 2004.

[32][33] Dissanayake's victory was largely attributed to the dissatisfaction of the previous governments amidst the nation's ongoing economic crisis.

[35] Tamil National Alliance MP M. A. Sumanthiran congratulated Dissanayake on what he claimed as "a victory without relying on racial or religious chauvinism, a key factor that set his campaign apart.

In his inaugural speech as president, he promised to fulfill the commitments listed in the mandate, reiterating that it would take time for the country to change.

[39] Due to the seat of Dissanayake being vacant in parliament, Lakshman Nipuna Arachchi was appointed as the former's replacement as MP for the Colombo district.

Dissanayake was highly critical of the Wickremesinghe government and the International Monetary Fund, claiming that the IMF only wished to bail out corrupt regimes.

[48] President Dissanayake instructed 11,000 acres of land belonging to the Kantale Sugar Company to be distributed to farmers for the cultivation of short-term crops.

[58] Also on 22 October 2024, Foreign Secretary Aruni Wijewardane led the Sri Lanka delegation to the 2024 BRICS summit in Kazan, Russia.

[62] Dissanayake visited the Archbishop of Colombo, Cardinal Malcolm Ranjith, where he received blessings and later vowed to uncover the truth surrounding the 2019 Easter Sunday bombings.

[63] Responding to calls by Northern Province residents, Dissanayake ordered the reopening of the Palali-Achchuveli main road in October 2024, which was carried out by the Defence Secretary.

The road which ran across the high-security zone of the Palaly Cantonment in Jaffna had been closed for over 30 years since the start of the Sri Lankan Civil War.

[64] Former Ilankai Tamil Arasu Kachchi parliamentarian M. A. Sumanthiran urged President Dissanayake to release all remaining military-occupied land, both residential and agrarian.

[67] Dissanayake has responded to the threat of terrorism that appeared in October 2024 with travel advisories raised by the United States and several other countries, by deploying the police, intelligence officers and the armed forces in Colombo, Arugam Bay, Weligama and Ella; and meeting foreign envoys.

[75] During his 2024 presidential campaign, Dissanayake pledged to dissolve the then-incumbent parliament within 45 days of coming to power and seek a general mandate for his policies.

[76] Dhananath Fernando, CEO of the Colombo-based pro-market think tank Advocata Institute, said that Dissanayake "now advocates for a pro-trade approach, emphasising the simplification of the tariff structure, improving the business environment, reforming tax administration, ending corruption and positioning the private sector as the engine of growth.