Rupert Murdoch's publishing group has been searching for a chief executive of Unruly to replace Sarah Wood, who stepped down along with her two other co-founders, Scott Button and Matt Cooke, last autumn.
Johnston has been in digital marketing since 1995 when he started work at an early digital agency, Modem Media, which later became Digitas London, where he was president.
He joined WPP's Mindshare in 2007 in an EMEA leadership role and is known as a thinker who has written several books, including Adaptive Marketing.
News UK acquired Unruly in 2015 just as Rebekah Brooks returned to the helm as chief executive of the publisher of The Sun and The Times.
Unruly was a darling of the London start-up tech scene and the acquisition was seen as a way for News UK to boost its expertise in online video and digital distribution, but it has also proven to be a challenging time as Google and Facebook have sucked up virtually all the growth in digital ad revenues globally.
Of Johnston's appointment, Brooks said: "Norm's data-driven approach and deep understanding of marketers' requirements will be invaluable to Unruly clients and the wider News Corp business. He's a programmatic pioneer with truly global experience and I'm very much looking forward to working with him as we build on the milestones that Unruly has already achieved.
"I am deeply grateful to Sarah Wood and to her co-founders, Scott Button and Matt Cooke, for their commitment to transforming advertising for the better. They have built Unruly into a scaled and trusted advertising platform and I look forward to Sarah chairing the Unruly board and continuing to bring her passion and vision to the table."
Johnston grew up in Ohio but lives in London.