Willem studied economics in Groningen, the Netherlands. After finishing an MBA in France, Willem started working at Merrill Lynch in London in 1998, within the investment banking division. He worked for various teams: Benelux, Energy & Power, M&A and also briefly in South Africa. In 2004 he set up his own M&A boutique, Sequoia; initially from London but he moved this business to the Netherlands in 2008. In 2008 he started MC Groep, together with Loek Winter, with the objective to manage hospitals. In 2009, Willem became executive board member of the IJsselmeerziekenhuizen, the first hospital acquisition of MC Groep and in 2014 of Slotervaartziekenhuis, its second hospital acquisition. Since the summer of 2016, Willem is CEO of MC Groep and its two hospitals. Willem is currently managing director of MDCA, a diagnostic laboratory in Amstelveen and board member of the NVZ, the Dutch Hospital Association. Willem is shareholder and former supervisory board member of DeSeizoenen, a company which cares for people with a disability and a supervisory board member of DOEN, which distributes funds received from the three main Dutch lotteries.