Nielsen replaces David Kyffin, who moved to the newly created role of global chief ROI and direct marketing officer at MediaCom Worldwide in November last year.
Nielsen moves from Microsoft, where he was the Western Europe sales director, and will be responsible for overseeing all Group M digital activity for WPP agencies in the UK including MediaCom, Mindshare, Mediaedge:cia and Maxus – reporting to Nick Theakstone, Group M's chief executive for the UK.
He joined Microsoft in 2002 as the sales and trade marketing manager for MSN Denmark and before joining Microsoft was a salesman at Business IT, a software company. He also founded two digital marketing companies as the managing director of QPeople A/S and Handsoffmedia A/S.
Theakstone said: "We are delighted to secure such a talented individual with such a broad skill set. Jakob will be a huge asset."