Carl Berendsen addressing the United Nations, 1946

Carl August Berendsen addressing the United Nations Assembly in 1946.

On 16 March 1944 Berendsen had been appointed, somewhat to his surprise, as New Zealand minister in Washington.  In 1948 the legation was raised to an embassy and Berendsen became ambassador. Although appointed initially for three years, he had his term extended by a year in 1947; this became a yearly ritual until 31 January 1952.

In Washington Berendsen, who was made a KCMG in 1946, represented New Zealand at numerous conferences. He sat on the Far Eastern Commission, which somewhat ineffectually oversaw the occupation of Japan, visiting Japan with the commission in 1945 and serving as chairman of the steering committee until 1949. He and Prime Minister Peter Fraser were New Zealand's delegates to the San Francisco Conference in April-June 1945; both signed the United Nations Charter on behalf of New Zealand on 26 June. Disappointed with the outcome, Berendsen was henceforth an implacable opponent of the great-power veto in the Security Council.

Between 1946 and 1951 he led the New Zealand delegation at meetings of the General Assembly in New York. Mainly on his own advice, he was appointed New Zealand's permanent delegate at the United Nations on 20 May 1949. From 1947 Berendsen also represented New Zealand on the United Nations Trusteeship Council; he was elected its vice president that year.