As companies in the reputationally challenged life insurance sector overhaul their business models, to serve customers rather than salesmen, industry reformer Mr McMahon has been in demand.
He says: "There is the potential for an industry revolution in the way personal financial services are delivered."
After leading modernisation at Axa, Mr McMahon moved across to the new Friends Life in late 2010, was lured to Edinburgh to join Aegon in early 2011, then in May 2012 was poached again, this time by FNZ, whose headcount of 250 mainly skilled graduates in Edinburgh has been rising by the month.
The New Zealand company boasts "a unique combination of market-leading platform technology and an investment administration service". FNZ is the power behind Standard Life's reinvention of itself, and other key customers include UBS, HSBC, Australian giants NAB and ANZ, and Mr McMahon's former employers Axa, Friends, and Zurich (where he spent 12 years).
But it was a trip to Scotland in 2007 that Mr McMahon recalls as a key pointer in his career. In a private speech at Gleneagles, the then Financial Services Authority chairman Callum McCarthy shocked his senior audience by signalling a consumer revolution. "He challenged the whole industry and said your model is broken, customers are being inadequately served," Mr McMahon recalls. "I have always believed that the fundamental things we do in financial services, the sort of protection we provide, long-term savings – because people are going to be in varying forms of retirement for 20, 30 or even 40 years – are fundamentally important to individuals and to the economy of the country.
"My frustration was that notwithstanding the credibility of what we do, the industry had failed to engage with its customer, there was opacity, lack of trust was endemic."
He adds: "There were like-minded people in a number of financial services organisations who wanted to address that. I spoke at the same conference the following year and was very supportive of what he had said.
"It is not about altruism, but equally we operate in an environment where the consumer expectation is set against a whole range of comparable sectors. We need as an industry to be dramatically more sensitive to that."
A politics graduate of Durham University with business degrees from Harvard and Lausanne, Mr McMahon emerged as a director at Axa UK where he created the Elevate wealth platform. "I built a new business model. Our driver was the end consumer's needs and goals and how they were best served. We needed a business partner who was similarly minded and had the skill-set capability: that is more difficult to achieve in large institutions."
The 49-year-old followed the trail to New Zealand, where Standard Life had in 2005 signed up a tiny institution – software start-up FNZ – to create the platforms and product wrappers at the heart of its new strategy.
Axa followed suit, and five years on it was FNZ which came calling for Mr McMahon, who had just decamped from his Wiltshire home to a flat in Murrayfield, as he got his feet under the table at Aegon.
"I had to wrestle with my conscience that I hadn't been at Aegon very long, I was very sensitive about that and took counsel, including from Adrian [Grace, chief executive of Aegon UK]," Mr McMahon says. "But the opportunity to be CEO of an organisation like this was very difficult to pass over, and they [Aegon] were good enough to give it their blessing."
FNZ is neither competitor nor customer for Aegon, which, however, suggested some gardening leave for its prized but short-lived sales and marketing director. It was, he says "an incredibly frustrating experience – I am not very good at sitting still and doing nothing, I sort of disappeared and went around down south, swung a golf club and dealt with some domestic things".
FNZ, bought out by its management with private equity backing in 2009, sees Mr McMahon as the ideal champion for its maturing relationship with the industry's key players. "We have got common cause in the modernisation agenda," the new boss says. "Fundamentally we have very clever software that supports all stages of the investment process, sitting on clever infrastructure."
Its administered assets have rocketed from £2 billion in 2007 to £30bn this month, for more than 500,000 end customers. Last January, First Minister Alex Salmond formally opened its Edinburgh base as the FNZ European headquarters. It has hired more than 200 graduates in the capital since 2010, and it employs around half of its 700 worldwide staff in Edinburgh and Bristol.
Mr McMahon says: "I suspect we could hire globally another 300 in this organisation in 2013."
He is looking at "linking in to how we put insurance under investment" on a platform, and at expanding the European dimension of the business, which employs the other half of its workforce in a development centre at Brno in the Czech Republic.
The McCarthy wake-up call has led eventually to the new era, about to dawn in January, of "adviser charging" rather than sales commission on financial products. For Mr McMahon now there is a real sense of the industry transforming itself, and cites the likes of Friends, JP Morgan and Zurich as being on the same page. "We can see it happening as we go around boardrooms," he says.
The well-travelled insider also admits: "It is not easy, there are a lot of existing habits."
Transferring assets such as a Sipp, for instance, from one platform to another, can take weeks, to the continuing exasperation of financial advisers. Mr McMahon agrees: "It can be done a lot quicker. It will end up being regulated if that doesn't happen."