Gardiner was born in 1914 in Kumasi, Gold Coast, into a family of successful merchants. After attending Adisadel College, he received his education in Sierra Leone and in the United Kingdom. Following the completion of his studies, Gardiner became a professor of economics at Fourah Bay College. From 1946 until 1948 he served as an area specialist for the UN Trusteeship Council. In 1949 he was hired as the first Director of Extramural Studies at University College in Ibadan, Nigeria. Four years later he returned to the Gold Coast to serve in the higher levels of the civil service. Following the country's independence as Ghana in 1957, Gardiner was appointed by Prime Minister Kwame Nkrumah to be Establishment Secretary. The two had a tenuous working relationship as Nkrumah's idealism conflicted with Gardiner's practical style of administration. Following his dismissal in 1959, Gardiner accepted his nomination as Deputy Executive Secretary of the UN Economic Commission for Africa (UNECA). In 1960 civil war erupted in the newly independent Republic of the Congo and Gardiner became heavily involved in UN efforts to resolve the situation, serving as a mediator between various factions and as a close adviser to UN Secretary-General U Thant on the matter.