Between 1957-1960, he joined the Ghana Foreign Service and was posted to the Ghana Embassy in Washington to become the Permanent Representative of Ghana to the United Nations (UN). From 1960-1966, Dr Nikoi was made the Alternate Executive Director of the International Monetary Fund (IMF) and between 1966-1968 became the Executive Director of the IMF.
Dr Nikoi was appointed Senior Principal Secretary of the Ministry of Finance between 6 January1969-February 1973. On 1 March 1973, he was appointed Governor and Chairman of the Board of Directors of the Bank of Ghana, within the same period was the Chairman of Ashanti Goldfields Company (AGC) and Chairman, Grains Warehousing Company.
He was retired in 1977 as the Governor of the Central Bank by the National Redemption Council (NRC) under the late General Ignatius Kutu Acheampong. During the Third Republic under Dr Hilla Limann and the People's National Party (PNP), Dr Nikoi was appointed Minister of Finance and Economic Planning. He left behind a widow, Mrs Gloria Amon-Nikoi, three children and two grand children.