Gar Tongtsen was born into the Gar clan, an important Tibetan family based in modern Maizhokunggar County.
According to Clear Mirror on Royal Genealogy, Tongtsen was dispatched as envoys to Licchavi Kingdom (in modern Nepal) together with Thonmi Sambhota by the emperor Songtsen Gampo. Amshuverma, who was the ruler of Licchavi, married Princess Bhrikuti to Songtsen Gampo. But the historicity of the princess is not certain because no reference to her has been found among the documents discovered at Dunhuang.
Tongtsen was dispatched to the Tang dynasty together with Dri Seru Gungton and Thonmi Sambhota in 640, requesting a marriage between the Tibetan emperor and a Tang princess. Taizong, the Tang emperor, agreed and married Princess Wencheng to Songtsen Gampo. Taizong appreciated his talent and promoted him to "Right Guard Senior General" (右衛大將軍), but was refused by him.
Later, the Lönchen Khyungpo Pungse invited Songtsen came to visit his fief. Tongtsen was sent there to set up the emperor's camp, but found it was a conspiracy. Tongtsen fled stealthily and told the conspiracy to the emperor. After the death of Pungse, Tongtsen was appointed as Lönchen.
Songtsen Gampo died in 650, and Mangsong Mangtsen succeeded. Tongtsen acted as regent because Mangsong was an infant. During this period, Tongtsen carried out a tax reform in 653, conducted a census in 654, and made the first law of Tibet in 655. Relations between China and Tibet began to sour during this period. Tibet conquered glo po (Ngari) and rcang rgya (Tsang) in 652, and finally 'A-zha (which was called "Tuyunhun" by Chinese people) in 663. Tibet held over the whole of the Tibetan plateau, and tried to seize the Western Regions of Tang China.
Tongtsen left in 'A-zha to defend against Tang China, and was dismissed by Mangsong due to his age. Not long after his successor Omade Lotsen was executed because the "guilty of rebellion", and the position turned back to him. Six years later, he died of neck cancer on his way back to Lhasa.
