Following is father's legacy, he sought nomination from the Sri Lanka Freedom Party for the 1970 general elections from the Nuwara Eliya electorate, but was reduced. He instead contested from the United National Party and was elected as a member of parliament under United National Party candidature and was one of the 18 UNP members in the parliament at a time when his party has faced an humiliating defeat. However, he was unseated in an election petition and faced suspension of his civic rights for seven years. That year a modification of the law by Minister of Justice, Felix Dias Bandaranaike to benefit Nanda Ellawala, allowed for Dissanayake to retain his civic rights and contest the by-election regaining his seat in parliament.
Minister
He comfortably secured his Nuwaraeliya-Maskeliya multielectorate seat in the 1977 general elections and was appointed as the minister of Mahaweli Development. Dissanyake spearheaded the Mahaweli Development Project. This was a huge project which was expected to take 30 years to finish. But due to Dissanayake's skills he managed to finish the project within 6 years. The Mahaweli Development Project is the largest development project conducted in Sri Lanka after independence. The whole country was affected by the project which focused on irrigation, hydro power generation, agriculture, town & country development. Further it was criticised by the time as to introduce sinhala population into Tamil's motherland. Majority of the power generated for the local consumption is generated from these hydropower plants while an overwhelming majority of the rice cultivation in Sri Lanka is conducted in these Mahaweli areas. He was the chairperson of the Sri Lanka Cricket Board in the early 1980s and helped his country to gain the Test Status in cricket. He played a pivotal role in signing the Indoo-Lanka Peace Accord in 1987. In 1989 he was re elected as an MP from Nuwaraeliya District securing the highest percentage of preferential votes obtained by any UNP MP. He was the Minister of Estate Development during Premadasa regime. He was not awarded a portfolio in 1990 cabinet reshuffle and remained as a back-seat MP in the parliament. Dissanayake was accused of abduction of Prof. Ralph Bultjens. Prof Rohan Goonarathne in an interview with Hasitha Kuruppu told that Dissanayake did that as Bultjens in his lectures accused Dissanayake for swindling of money. Later he was acquitted from the courts.
He played a major role in the aborted impeachment motion against President Premadasa and as a result was expelled from UNP in 1991. In the same year Dissanayaka joined Lalith Athulathmudali to form United National Democratic Front which was a rising political force of that era. Dissanayaka contested to Kandy District at 1993 provincial council elections and became a member of the Central Provincial Council. After the assassination of Athulathmudali, Dissanayake became the leader of DUNF.
Return to the UNP
Later he rejoined UNP and was appointed to the parliament as a national list MP and was offered a vital portfolio in the Wijethunga Government. In 1994 PA came to power defeating UNP when Dissanayaka was re elected as a member of parliament from Kandy District. In a contest held among UNP MPs to choose the opposition leader Dissanayaka recorded a comfortable victory over Ranil Wickramasinghe.
Gamini Dissanayake married Srima Lenaduwa in 1968. They had two sons, who are active in politics, Navin and Mayantha. His eldest son Navin was the Cabinet Minister of State Management Reforms in President Mahinda Rajapakse's Government and Member of Parliament for Nuwara Eliya District; he was non-cabinet Minister of Investment Promotion too. Mayantha contested for the 2010 General Election for Kandy District as a UNP candidate.