Mr. Wickramanayaka served as Prime Minister on a previous occasion too (August 2000-December 2001) when he succeeded the late Mrs. Sirimavo Bandaranaike ( first woman Prime Minister of the world ) after she resigned from the position at the age of 84. He is a senior Vice-President Sri Lanka Freedom Party (SLFP) which is the main constituent party of the ruling coalition. He is the most senior Parliamentarian still active in politics in Sri Lanka.

Ratnasiri Wickramanayaka was educated in Millewa Primary School, Dharmapala Vidyalaya Pannipitiya and the prestigious Buddhist school, Ananda College in Colombo. Later he proceeded to London to join Lincoln's Inn to read for the Bar. But he was obliged to curtail his studies and return to Sri Lanka following the sudden death of his elder brother, Munidasa who was in active politics in their birth place of Horana. He was elected President of the Ceylon Student's Association in the United Kingdom in 1955. He was an eloquent speaker and public debater from his student days, and has continued to retain this talent.

On his return to Sri Lanka, he was elected to Parliament in March 1960 from the Mahajana Eksath Peramuna (Peoples' United Front) to represent the Horana constituency. He was re-elected in July 1960, as an Independent. Mr.Wickramanyaka joined the Sri Lanaka Freedom Party(SLFP) in 1962. In 1965, and in 1970 he was re-elected to the legislature for Horana from the SLFP.

Mr.Wickramanayaka received his first ministerial appointment in 1970 , when he was appointed Deputy Minister of Justice in the United Front government under Prime Minister Sirimavo Bandaranaike . In 1975 Mr.Wickramanayaka was appointed Minister of Plantation Industries and in the following year he was made, in addition, the Minister of Justice. Like many other party stalwarts, Mr.Wickramanayaka too lost his Parliamentary seat at the election held in 1977 . He became General Secretary of the SLFP in 1978 .

He was given a senior position in the government of President Chandrika Bandaranaike Kumaratunga. becoming Minister of Public Administration, Home Affairs and Plantation Industries in 1994 , and was also named the leader of the SLFP parliamentary group and Leader of the House.

Following the UPFA victory at the 2004 Parliamentary Elections, Mr.Wickremanayaka was appointed Minister of Buddha Sasana, Public Security, Law and Order, and Deputy Minister of Defence. In August 2005 Mr. Wickramanayaka was allocated yet another portfolio, that of Agriculture on the resignation of the incumbent Minister. He held all these positions till he was appointed Prime Minister in November 2005 by the newly elected President Mahinda Rajapaksa. In addition to the premiership he also holds the portfolio of Minister for Internal Administration. He continues to be Deputy Minister of Defence.