[1] McIntosh joined the New Zealand the public service as a cadet in the Department of Labour's head office in March 1925, where he was employed in the library.
In July 1926 he transferred to the Legislative Department after taking a position as an assistant librarian in the General Assembly Library.
[1] After being awarded a Carnegie Fellowship and he was granted in 1932 a year's leave of absence to study library development and archive procedure in the United States and Canada.
[2] McIntosh worked and was close to such intellectuals as Dr William Sutch, Frank Corner, and eminent historian J.C. Beaglehole.
[1] During the war years, McIntosh chaired the economic stabilisation committee, and worked closely with Prime Minister Peter Fraser.
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.
Following the end of World War II McIntosh, despite restrictions on government expenditure, commenced building up and staffing the Department of External Affairs.
[3] He retired as Secretary of Foreign Affairs in 1966 (he was succeeded by George Laking), but then established New Zealand's posting in Italy, serving as ambassador there until 1970.
Ngaio Marsh, in her 1974 mystery novel Black As He's Painted, thanks McIntosh (and P. J. Humphries) for advice on matters ambassadorial and linguistic related to an African embassy in London.