Mr. Sorrell’s reputation as an exacting, exhausting boss has not daunted his new partners. In a recent interview, MediaMonks’ chief executive, Victor Knaap, said that due diligence research into Mr. Sorrell had turned up praise for his acumen and for his unparalleled network of contacts. As to the lore about his manic pace, if anything, the pace is more manic than Mr. Knaap had anticipated.

“Martin said in an interview after we announced the deal that we’d have a nanosecond to enjoy it,” Mr. Knaap said. “I’m still waiting for that nanosecond.”

A zeal for control

If the story line of “Mad Men” had stretched into the late 1980s, there would have been a character based on Martin Sorrell. He would have been a coiled, determined and thin-skinned British man in his early 40s — part pugilist; part charmer — and he would have incensed ad agency honchos by acquiring their companies in hostile takeovers. Four real-life agencies mentioned in the show — Young & Rubicam, Grey, Ogilvy and J. Walter Thompson — were eventually snapped up by WPP.

It was not the first advertising holding company, but WPP was the most aggressive and, eventually, the largest. Mr. Sorrell helped rationalize a business that had long been far from rational, to the great benefit of shareholders and to the abiding chagrin of a lot of ad creators. He helped take the crazy out of advertising, they say, imposing fiscal discipline on a realm that had long accommodated the freewheeling, unquantifiable magic of creativity.

It is hard to control what cannot be quantified, and Mr. Sorrell’s career has been defined by a zeal for control. That was evident during two, hourlong interviews for this article. Before commencing the first, he demanded to review his quotes, pre-publication, along with the context in which they would be presented. When it was explained that The New York Times would not agree to such conditions, he did not budge. A negotiation ensued. Ultimately, he agreed to talk but only if his comments were paraphrased.

With his blue suit and neatly trimmed gray hair, Mr. Sorrell looks like the guy in a James Bond movie who shows up and explains how the weapons work. Actually, Mr. Sorrell swears he is so mechanically inept that he cannot change a light bulb. Although known as a titan of the ad industry, he rarely discusses the creative aspect of campaigns. Deals are his passion: One of his go-to stories involves convertible preferred stock. He also has a gossip’s love of inside chatter. In conversation, he is expansive, sometimes peevish and occasionally funny.