He was appointed as a monastic prime minister, along with Lukhangwa, a senior lay official, by the Dalai Lama before he left for Yatung in the Chumbi Valley in December 1950 following the Chinese invasion of Tibet.

In his autobiography Freedom in Exile, the Dalai Lama writes that with the agreement of Lukhangwa, Lobsang Tashi and the Kashag, he sent delegations to the United States, England and Nepal in the late 1950s in the hope of an intervention for Tibet, as well as in China to negotiate its withdrawal.

From Chamdo, Ngapoi Ngawang Jigme sent a long report to the Tibetan government explaining that unless an agreement was reached, Lhasa would be attacked by the People's Liberation Army (PLA), resulting in many deaths.

The Dalai Lama therefore sent Ngapoi to Beijing with two personalities from Lhasa and two from Yatung, hoping that he would make it clear to the Chinese authorities that the Tibetans did not want "liberation", but only the continuation of good relations with China.

In his book The Fire in the Snow, Palden Gyatso mentions that when he arrived at Drapchi Prison in 1964, Lobsang Tashi was incarcerated there in the 5th brigade which housed former Tibetan government officials and high lamas.