In 1966, Sen was appointed the district secretary of the
Students Federation of India, students wing of the party. He played a key role in expanding the Party in Burdwan district, and served as the secretary of the
Bardhaman district Committee from 1989 to 1995. In
1987 West Bengal Legislative Assembly election, he was first elected as MLA from Bardhaman town assembly constituency. He was elected to the West Bengal state committee in 1985 and he became a member of the state secretariat in 1995. He was elected to the Central Committee in 1998. He became a member of the Polit Bureau in 2008. Sen was elected to the West Bengal State Assembly in
2001 and
2006 from the
Bardhaman Dakshin. After the Left Front was voted to power in 2001, Sen was handed the charge of Commerce and Industries and continued until 2011. He was considered to be the prime mover of the industrial drives in West Bengal. Under the leadership of him and then Chief Minister Buddhadeb Bhattacharya,
Left Front Government led new industrialisation policy in
Singur and
Nandigram. He stepped down from the central committee and the politburo in 2015 and from the state committee in 2018 due to ill health. However, he became a special invitee to the Central Committee between 2015 and 2018. ==Controversy==