As a captain posted to the Chitral region of Pakistan, Streather was the official government representative in a Norwegian expedition that made the first ascent in 1951 of Tirich Mir (7,710 m), the highest mountain of the Hindu Kush. He was initially appointed transport officer, but became a member of the successful summiting team. On his return to Britain he was invited to join the Alpine Club (he later became its president), and was selected for trials for the 1953 British Mount Everest expedition. He was rejected for the latter because of a lack of technical experience, but nearly at the same time was invited to join the 1953 American Karakoram Expedition, which attempted a far more technical route up K2, the second highest mountain in the world. Though again originally in charge of logistics, he climbed as high as anyone else and was involved in the dramatic events at 7,800 m.