Norman, 57, said media agencies need to be "on a war footing" at a time of great disruption and he is retiring from his global role in January to make way for "young generals".
He will become a part-time adviser to Group M and plans to stay involved in the digital world because he has been "so obsessed professionally" with Google and Facebook and "the evolution of media".
Norman has spent 31 years working for media agency CIA, its subsequent incarnation MEC and parent company Group M and played a leading role in their growth on both sides of the Atlantic.
"Group M, WPP and before that CIA have been my work, my life and almost always my pleasure," he said. "My friends in the business know that I’ve become very attached to a rural life and understand that after 31 years, it’s time to do other things."
Norman, who is British but moved to New York in 2005, likened the current challenges facing media agencies to the "dark ages" of the "Black Monday" sterling crisis of 1992, the dotcom crash of 2000 and the collapse of Lehman Brothers in 2008.
Those past crises were "macro" shocks that "affected entire economies and every sector" whereas the current wave of digital disruption is affecting the media industry and fast-moving consumer goods firms, in particular, he said.
All of the big ad groups, including WPP, have reported a slowdown in revenues this year as FMCG firms such as Procter & Gamble and Unilever, the world's top two advertisers, have reduced agency fees.
"There has been a significant co-dependence between the building of packaged goods brands and high-reach, passively-attended-to media and that contract is clearly not what it was," Norman said.
"Media agencies need to think about the world now in the same way they needed to think about it in the darkest days of post-Lehman or any other recessionary period they can remember and put themselves on a war footing," he declared.
"What a war footing means is you make decisions more quickly, that you look very, very hard at what your available assets are and make more use of those assets. It’s when a shovel becomes a weapon. You go to the ‘core principles’ of what your business is about."
Norman said that it was essential to promote a new generation of younger talent.
"What war-time does is it makes for young generals," he said. "You look really hard into your talent pool and instead of saying, ‘we’ve got 1,000 graduates and who’s going to emerge first?’, what you do is you start plucking them out earlier and earlier and fast-tracking the good ones.
"You need to create that active aggression and momentum amongst the people that work for you because the thing you can’t have them believing is that you somehow are hanging onto an [old] idea. It’s not that the idea has had its time but that it needs to evolve."
He added that some of the ad industry’s current crop of leaders "have to pause and say, ‘it’s not about me, me, me – this is about the next generation and the core functionality of the business".
Norman said he had "always been planning to stop between 55 and 58" because he can afford to retire and has just finished building a home in West Cornwall, Connecticut.
"I’m still very emotionally invested in the business," he said, explaining how he will continue to advise a handful of Group M’s clients and its global chief executive, Kelly Clark. "I do believe in younger generals but I believe in staying close and helping influence."
Norman first made his name at Chris Ingram’s Tempus Group, which owned CIA and was listed on the London stock market until its acquisition by WPP in 2001.
He was a relatively young general, joining the PLC board of Tempus at the age of 39, where he sat alongside other rising stars who were of a similar age.
These included Mainardo de Nardis, who later ran Omnicom’s OMD Worldwide, and David Wheldon, who is now chief marketing officer of Royal Bank of Scotland.
Following the sale of Tempus, Norman became UK chairman of a new, enlarged agency, Media Edge: CIA, which later became known as MEC, in 2002.
He became chief executive of Group M Interaction in New York in 2005, chief executive of Group M North America in 2011 and global chief digital officer in 2012.
That decade between 2002 and 2012 was a heady time when media agencies moved from being national to global businesses.
Norman worked with other WPP chiefs, including Irwin Gotlieb, Dominic Proctor and Sir Martin Sorrell, to build Group M into the dominant player with close to $100 billion in annual billings.
However, Group M and other media buyers have seen their power eclipsed by the tech giants in the last few years.
Norman said some of the highlights from the early years of his career were launching CIA Interactive, a pioneer in digital technology such as CD-ROMs, in 1994 and setting up the Euro96 website for the European football championships in England in 1996.
His other highlights include launching Media Edge: CIA and winning AT&T’s business in America – an account that Group M held until last year.
Norman said the investigation by the US Association of National Advertisers into "non-transparent" practices at media agencies in 2016 was "the thing that upset me the most" because of what he described as the "still lacking smoking gun".
He said he felt fortunate to "step back, yet stay involved" at Group M, following his retirement.