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".