On his return he worked closely with Prime Minister Peter Fraser as he developed a more distinctly New Zealand position within the framework of the Commonwealth.

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.

Corner was considered to be very close to Prime Minister Norman Kirk in arguing and advocating for an independent New Zealand foreign policy.

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.