After a period as a postdoctoral fellow at the University of Maryland, College Park in 1953, Mazur became an associate professor at Leiden University in 1954. In 1955, he and de Groot, who had also moved to Leiden, founded the Lorentz Institute for Theoretical Physics at Leiden University. In 1961, Mazur became a full professor, and when de Groot left in 1963, he became director of the institute. He filled this position in his own distinctive way for 25 years until he became emeritus in 1988. Under the direction of de Groot and Mazur, the institute grew substantially and eventually established the Lorentz Chair, a prestigious special professorship.