Subba, who started student politics from 2042 BS, got associated with CPN-UML in 2069 BS. He left the party the same year and became senior vice-chairman of Sanghiya Samajwadi Party. He rejoined UML in 2071 BS and became a politburo member. Subba was elected central committee member of the party in 2075 BS.
He was directly elected from Terhathum in polls held in 2048 BS and 2056 BS. Subba was elected in the last polls through the proportional representational system.