Thorkil Kristensen was elected to the Danish Parliament 1945 and became finance minister under Knud Kristiansen (1945–1947) and Erik Eriksen (1950–1953). Throughout his life he worked with difficult economic problems. Among people of his own party and opposing parties, he enjoyed great respect because of his broad knowledge of economics.
He came to disagree on economic policy with his party, Venstre, and left the party in 1960.
After his exit from politics, he was secretary general of the OECD from 1960-1969. He was the founder of the Copenhagen Institute for Futures Studies (CIFS), making it one of the first futures research institutes on the European continent. He was managing director at CIFS from 1970–1988.
He participated in the Club of Rome which attracted considerable public attention with its report, Limits to Growth, which has sold 30 million copies in more than 30 translations, making it the best selling environmental book in world history.