In the same year, he was appointed head of the newly established Department of External Affairs, the precursor to the Ministry of Foreign Affairs. He accompanied Fraser to the Australia–New Zealand meeting in January 1944 and later that same year to the 1944 Commonwealth Prime Ministers' Conference in London. McIntosh was a senior member of the New Zealand delegation to the San Francisco Conference in 1945, which was to lead to the establishment of the United Nations.
In that same year, he became the head of the newly established Department of External Affairs, which eventually evolved into the Ministry of Foreign Affairs. He accompanied Fraser to the Australia–New Zealand meeting in January 1944 and then participated in the 1944 Commonwealth Prime Ministers' Conference in London. McIntosh was a senior member of the New Zealand delegation at the San Francisco Conference in 1945, which led to the formation of the United Nations.