 For several years after leaving the university, De Groot taught mathematics at the secondary school level, but in 1946 he was appointed to the Mathematisch Centrum in Amsterdam, in 1947 he began a lecturership at the University of Amsterdam, in 1948 he moved to a position as professor of mathematics at the Delft University of Technology, and in 1952 he moved again back to the University of Amsterdam, where he remained for the rest of his life.He was head of pure mathematics at the Mathematisch Centrum from 1960 to 1964, and dean of science at Amsterdam University from 1964 on.He also visited Purdue University (1959–1960), Washington University, St. Louis (1963–1964), the University of Florida (1966–1967 and winters thereafter), and the University of South Florida (1971–1972).