Born in the Dutch capital city, The Hague, Casimir studied at the universities of Leiden, Copenhagen, and Zurich, and held various research positions between 1933 and 1942.

He has published many papers in the fields of theoretical physics, applied mathematics, and low-temperature physics. His most notable work has been in the theory of the superconducting state. Following the work of W. Meissner, Casimir and his colleague Cornelis Gorter advanced a two-fluid model of superconductivity in 1934 in which a fraction of the electrons were regarded as superconducting, while the rest remained ‘normal’ electrons. They were successful in explaining the high degree of interrelationship between the magnetic and thermal properties of superconductors.

From 1942, Casimir has pursued a highly successful career with the Philips company, becoming director of the Philips Research Laboratories in 1946, and a member of the board of management (1957–72). He has supervised Philips's research activities in several countries.