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.[7] On his return to Britain he was invited to join the Alpine Club (he later became its president from 1990–93), and was selected for trials for the 1953 British Mount Everest expedition.[7] 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.[8]
In 1955, Streather participated in the 1955 British Kangchenjunga expedition, the third highest mountain in the world. With Norman Hardie, he reached the summit the day after the first summit party of Joe Brown and George Band. This made him the first man ever to climb two peaks of over 25,000 feet (7,600 m).[9] In 1957 he survived an epic near-ascent of Haramosh (7,397 m).[10] In 1959 he led a successful expedition that included an ascent of Malubiting Southeast (6,970 m).[11]
Streather married his wife Sue (nee Mary Huggan) in April 1956, and they had a daughter and three sons.[13] Streather lived in his later years in the village of Hindon, Wiltshire.[9] He died on 31 October 2018 at the age of 92. Sir Chris Bonington spoke at Streather's memorial service in Hindon Parish Church, and his ashes were interred in the churchyard.