In 1961, Corner became New Zealand's Ambassador to the United Nations, where he chaired the Trusteeship Council for two years, and served on the United Nations Security Council. Corner was the leading proponent of decolonisation of Pacific territories under New Zealand control. Corner provided the leading intellectual arguments in favour of decolonisation of Tokelau, the Cook Islands, Niue, and Western Samoa. In 1967, Corner moved to Washington, as New Zealand's Ambassador to the United States, succeeding
George Laking, who had become Secretary of Foreign Affairs; he held this post until 1972 when he returned to Wellington to succeed Laking again as Secretary of Foreign Affairs and the Prime Ministers Department (the two roles were separated on his retirement in 1980). The principal themes during his time in Washington was the US' involvement in Vietnam, and New Zealand's expanding trade relations with the US leading up to Britain's entry into the European Economic Community. During his time as Ambassador, Corner oversaw a state visit to New Zealand by President
Lyndon Johnson in 1967, and a state visit by Prime Minister
Keith Holyoake to Washington. Corner later described this as "the high point of United States-New Zealand relations". Corner was considered to be very close to Prime Minister
Norman Kirk in arguing and advocating for an independent New Zealand foreign policy. Throughout this period, Corner promoted closer political relations with
China, an expansion of New Zealand interests in North and Southeast Asia, and a greater role for New Zealand in the South Pacific. The
Third Labour Government elected at the end of 1972 was expected to recognise the PRC. Then, surprisingly the new Prime Minister,
Norman Kirk, hesitated:
After barely two weeks in office he suggested that it might be better to delay recognition until his second term. However he was talked out of this by the Foreign Ministry (now under Frank Corner) and the recognition formalities were completed just before Christmas. ==Later life and death==