Koirala joined student politics in 2027 BS through the Nepal Students Union, a student wing of the Nepali Congress.
He received the party's membership in 2028 BS and became a central committee member through the eleventh, twelfth and thirteenth general conventions of the party. Koirala was defeated by Sher Bahadur Deuba in the elections for the party's president in the fourteenth general convention.
He was elected a lawmaker from Morang-6 in by-elections held in 2065 BS and was re-elected in the second Constituent Assembly elections from Morang-7 held in 2070 BS.
