In the early 1970s he was one (along with his brother, R.K. Mainali) of the radical communists who led the Jhapa rebellion, inspired by the Naxalite movement in India.

[4] In 1971, Mainali was one of a group of young leaders of the Jhapa District Committee of the Communist Party of Nepal.

The party's leadership did not approve of the methods used by the Jhapa movement, and Mainali and his fellows were now an independent grouping.

In 1975, the survivors of the Jhapa movement took the initiative to found the All Nepal Communist Revolutionary Coordination Committee (Marxist-Leninist).

On December 26, 1978, ANCRCC (ML) organized the founding congress of the Communist Party of Nepal (Marxist-Leninist).

CPN (ML) was an underground party, and conducted small-scale, armed activities against the regime and feudal landlords.

Mainali, clearly identified with the initial militant phase of the party, was removed from his post of general secretary and replaced by Jhala Nath Khanal.

[7] Mainali and Bam Dev Gautam led a break-away group that was named Communist Party of Nepal (Marxist-Leninist).

[9] At the time of the 2000 CPN (ML) party congress, Mainali challenged Bam Dev Gautam for the post of general secretary.