Robert Senior, who takes over from Kevin Roberts as worldwide CEO of Saatchi & Saatchi in January 2015, describes Mr. Roberts as "the best boss I ever had. When you need him, he's there immediately; otherwise, he allows you to operate in the way you want to operate."

But that doesn't mean Mr. Senior – currently chairman and CEO of SSF Group Europe Middle East & Africa, as well as chairman of the worldwide creative board -- isn't planning to do things differently than his predecessor when a new generation of Saatchi management moves up next year. He is reluctant to talk specifics before he is in the job, but he is nevertheless clear that there is work to be done, and hints that things will change from January. (That's when Mr. Roberts will move to the newly-created post of chairman, and also take on the role of "head coach" of Publicis Groupe until 2017, when he and Publicis Chairman-CEO Maurice Levy are due to retire).

"I'm going into this with my eyes wide open," Mr. Senior said. "I'm determined to give it the best crack possible. It's a tough gig but it's one I believe in. There are a lot of pressures out there – the market is going through so much change – but if there's one thing I've learnt, it's that it all starts with belief. Then it's about getting the right team and being focused."

Mr. Senior was speaking from London's Heathrow Airport, where he was waiting to board a flight to Miami to meet with Pablo Del Campo, Saatchi's worldwide creative director since March 2014.

The two men are meeting to review work and discuss "talent opportunities," Mr. Senior said. "Our partnership has been incredibly important over the last 18 months. It's a fundamental part of our future, working on delivering against the [Saatchi & Saatchi] brand promise. Pablo's amazing. We're doing everything we can to make the work as good as it can be, everywhere, and on getting the right creative leaders and giving them the right responsibilities and support. We have pockets where the agency's amazing, and pockets where it could be better."

To expand one of the best pockets, Mr. Del Campo's own Del Campo Saatchi & Saatchi agency in Argentina, he opened an agency in Spain two years ago under the Del Campo Saatchi & Saatchi brand to improve work for clients like Toyota and win new business. As Mr. Del Campo takes on new responsibilities along with Mr. Senior, his two executive creative directors from Argentina, Maxi Itzkoff and Mariano Serkin, have been dispatched to Europe to provide creative leadership for the region.

Mr. Senior points to the Toyota work in Europe, which he says has recently become "night-and-day better – more interesting, more ambitious and more cohesive," and he promises that a new HSBC spot is "bloody brilliant." He adds, "I've always orbited the work. It's the only currency I really believe in in our business."

However, he said, "There's still heaps to do." Including, he admitted, "reputation stuff" to deal with at Saatchi New York, as well as new business (or lack thereof) to address. He insisted that New York has "perked up markedly" thanks to Jay Benjamin, chief creative officer, and CEO Brent Smart, who together are "like rocket fuel. They are building a team with similar passion and belief."

From January, Mr. Senior expects to spend about half his time in the U.S., but he will remain based in London, where the Saatchi & Saatchi story began back in 1970 under founders Maurice and Charles Saatchi.

"The U.S. is our largest market and it's the biggest new business market in the world," he said. "I'm going to put a lot of effort into helping new business, and there's a lot of opportunity to embrace new platforms. I've got a bunch of plans for that."

Will his lack of U.S. experience – Mr. Senior has only ever worked out of London – hold him back? "I haven't taken the train out to New Jersey every day, but I've spent an awful lot of time in the U.S. with clients," hesaid, "and I've been part of a lot of global conversations."

The third man in the new-generation triumvirate at the top of Saatchi & Saatchi from January will be Chris Foster, currently chairman and regional CEO of Saatchi & Saatchi Asia Pacific, who becomes worldwide chief operating officer, reporting to Mr. Senior. The two men worked together (albeit from opposite sides of the Atlantic) when they were at Fallon, and both sit on the worldwide leadership team. Besides valuable experience running Asia, Mr. Foster, a Canadian, has the U.S. background Mr. Senior lacks, after a three-year stint as CEO of Fallon in Minneapolis before moving to Singapore in 2011. He also forged deep ties to Saatchi's biggest client from his days as global equity director for Procter & Gamble brands.

"We are very different," Mr. Senior said. "If we're smart, we'll play to each other's strengths."

So, what are Mr. Senior's strengths? He's famous for his straight-talking (to clients as well as staff), for fighting the creative corner, and for relentless hard work combined with ruthless ambition. His energy and enthusiasm for the business help make him a natural leader.

Mr. Senior studied politics and history and chose advertising as a career after he went to a recruitment event, where an Ogilvy creative director showed two campaigns and asked the audience which was better.

Everyone chose the same work, apart from Mr. Senior. But when the creative director told him he was right, Mr. Senior saw that he had a natural instinct for advertising -- and has since made a career out of never being afraid to go against the grain.

He went to DMB&B (now part of Leo Burnett) to work on Procter & Gamble, before moving on to Simons Palmer Denton Clemmow & Johnson (now part of TBWA). It was at Simons Palmer that he worked for founder Carl Johnson (who later went on to set up Anomaly). "Carl has a focus like no one else," Mr. Senior said, "and he's very candid in his feedback. I always try to be clear and fair and consistent, which are Carl traits. Everything has perspective and context."

Mr. Senior and four partners then founded Fallon London in 1998, creating an agency that made some of the most memorable work of the last decade – including "Balls" for Sony Bravia TVs, Cadbury Dairy Milk's "Gorilla" and Skoda "Cake."

"I'm very conscious of how lucky I was to work with that group of people," Mr. Senior said. "It was a real adventure and a great example of playing in position. The potency of a team is extraordinary."

Outside work, Mr. Senior's interests mostly revolve around mountains. He has a business renovating alpine farmhouses in France; he is a qualified ski instructor (he taught in Austria, Switzerland and France to help pay his way through university), and he likes nothing better than to climb mountains and then ski back down them across dangerous off-piste routes.

It could be argued that he applies the same maverick -- and sometimes impetuous -- spirit to his working life. Although he says he feels "ridiculously honored and humbled" by the opportunity to lead Saatchi & Saatchi, that doesn't mean he is overawed by the challenge. "I have a mountain to climb," he said. "Luckily I like mountains."