Thapa, who is the son of former Prime Minister Surya Bahadur Thapa, entered politics in 2011 after resigning from UNHCR. He was elected in the second Constituent Assembly elections in 2070 from Dhankuta-2 representing the Rastriya Prajatantra Party. He became Minister for Industry, Commerce and Supplies in the Sher Bahadur Deuba-led government in 2047 BS. Thapa, who was a central committee member of the Rastriya Prajatantra Party, entered Nepali Congress in 2077 BS and was appointed the parry's Joint General Secretary. He was elected central committee member of the party through the fourteenth general convention.
Sunil Bahadur Thapa