During his study in 9 grade, Although thapa was not politically inclined, he was introduced to the political chaos in 1990 when he participated in the street protest against the monarchial System.
His political career came out of his involvement with Nepal Students Union during his study at Tri-Chandra Campus.
He was elected the president of Free Students Union for the duration 1998-2000, he also became one of the elected member of central NSU committee member for 1998-2000, vice president for 2000 – 2002. Later in the year of 2002 he became the general secretary of NSU till 2004.
He was jailed numerous times after the February 1, 2005 royal takeover on charges of sedition against the state, for his outspoken stance against King Gyanendra’s regime. Amnesty International had declared him a prisoner of conscience, since he had been detained despite his peaceful & legitimate activities in support of democracy & human rights in Nepal.