From 1951 to 1969 he was the highest official of the Dutch Ministry of Finance. From 1969 to 1984 he was secretary general of the Organisation for Economic Co-operation and Development (OECD). At the OECD Van Lennep is remembered for making the organization a more effective forum for international cooperation. In the seventies he was named several times to be a candidate for a post in the government of the Netherlands, but he did not want to be a politician. After his career at the OECD he was appointed to be Minister of State from 1986 until his death in 1996.