Cornelis Marinus Leendert (Kees) Molkenboer was born on 7 April 1907 on Crooswijkseweg in Rotterdam. His father worked as a stable hand in a livery stable in the centre of town and later got a job as medicine deliveryman for a pharmacy. After primary school, Kees became an apprentice in the butcher's shop of Haak on Goudserijweg.
With the help of a supermarket savings scheme, Molkenboer bought his first camera and became more and more obsessed by photography. He joined photo club De Maasstad, which helped him to develop his technical skills.
In 1936 Molkenboer entered the trade register of the Chamber of Commerce under the name of International Photopress Office (IPO). He also opened a studio on Mauritsweg called Studio 36, where until 1939 he concentrated on a broad range of photographical activities, including portraits and architectural photography.
After the war, Molkenboer photographed at his own expense, selling his photographs to newspapers and journals in Rotterdam. In the first few years after the war this amounted to seventeen periodicals. During this period, press photographers met every month in café De Toog, where they set up the Rotterdam branch of the Nederlandse Vereniging van Fotojournalisten (Dutch Society of Photo Journalists). Molkenboer became their first chairman.
As a result of his contacts with various left-wing artists in the first post-war years, Molkenboer frequently worked for the communist newspaper De Waarheid. The archive includes many unique photographs from this period, documenting demonstrations against the Marshall Plan and against the war in the Dutch East Indies. Later he also worked for the Christian newspaper De Rotterdammer. From 1958 until 1968 he taught two days a week at the School voor Fotografie in Den Haag.