• 1938 – Joined the State Congress
• 1940 – Became member of the Communist Party of India
• 1952 – Appointed division secretary, CPI (Alappuzha division)
• 1956 – Took charge as district secretary, CPI (Alappuzha district)
• 1957 – Inducted into the state secretariat of the CPI (Kerala)
• 1958 – Nominated to the National Council of the Communist Party of India
• 1964 – Became Central Committee member, CPI(M)
• 1967 – Elected as MLA from Ambalappuzha constituency for the first time
• 1970 – Re-elected as MLA from Ambalappuzha constituency for the second time
• 1980 – Took over as state secretary of CPI(M) Kerala
• 1985 – Elevated to the Politburo of CPI(M)
• 1991 – Elected as MLA from Mararikulam constituency for the third time
• 1992 – Became Leader of Opposition in Kerala Assembly for the first time
• 2001 – Elected as MLA from Malampuzha constituency for the fourth time
• 2001 – Became Leader of Opposition in Kerala Assembly for the second time
• 2006 – Re-elected as MLA from Malampuzha constituency for the fifth time
• 2006 – Sworn in as Chief Minister of Kerala
• 2011 – Elected as MLA from Malampuzha constituency for the sixth time
• 2011 – Became Leader of Opposition in Kerala Assembly for the third time
• 2016 – Re-elected as MLA from Malampuzha constituency for the seventh time
• 2016 – Appointed Chairman of the Kerala Administrative Reforms Commission
