Luns was born in a Roman Catholic, francophile and artistic family. His mother's family originated from Alsace-Lorraine but had moved to Belgium after the annexation of the region by the German Empire in 1871. His father, Huib Luns, was a versatile artist and a gifted educationalist who ended his career as professor of architectural drawing at the Delft University of Technology. Luns received his secondary education in Amsterdam and Brussels. He opted to become a commissioned officer of the Dutch Royal Navy but registered too late to be selected. Therefore, Luns decided to study law at Amsterdam University from 1932 to 1937.