Lewenhaupt was embassy counsellor in Washington, D.C. from 1960 to 1962 and ambassador in Leopoldville from 1962 to 1963. Lewenhaupt was acting head of the political department at the Foreign Ministry from 1964 to 1965 and its administrative department from 1965 to 1967. He was ambassador in Bangkok, Rangoon, Kuala Lumpur and Singapore.