The Chamling-led Sikkim Democratic Front (SDF) had secured a two-thirds majority in the Assembly elections winning 22 seats out of 32.
He was yesterday elected as the SDF Legislature Party leader following which the Governor invited him to form the government.
In power since December 12, 1994, the 63-year-old Chamling has already completed two uninterrupted decades as the CM of the erstwhile Himalayan kingdom which had merged with India in 1975.
After being sworn in for the fifth consecutive time, he was on his way to break the late Communist patriarch Jyoti Basu's record of being India's longest serving chief minister. Basu had ruled West Bengal for 23 years from 1977 till 2000.