in 1950, Trench became Assistant Secretary to the Deputy Defence Secretary for Hong Kong. He eventually held the office of Deputy Financial Secretary in 1956 and Commissioner of Labour and Mines in 1957. In 1958, Trench studied at the Imperial Defence College in London.
Trench served as Deputy Colonial Secretary of Hong Kong between 1959 and 1960. He left Hong Kong to take up the office of High Commissioner for the British Western Pacific Territories between 1961 and 1964, only to be appointed Governor and Commander-in-Chief of Hong Kong between 1964 and 1971.