De Jong was most known for his work 'The Kingdom of the Netherlands during World War II', a 26 volume series of books describing the war year 1940-1945 in the Netherlands. The work was supposed to take only 6 years, but got completed only in 1988: together the books weigh 25 kilograms, and contain 15,000 pages.

Being Jewish himself, he had to escape the Netherlands during the war, to London. After the war he was the director of the State Institute for War Documentation, from 1945 to 1979. He became to symbolize everything having to do with the history of the war.
The Institute is now called