The Glasgow-born accountant, 61, is vice chairman of International Management Group, the massive sports and entertainment group founded by Mark McCormack, who represent global sporting superstars.
Johnston has been with the company almost 40 years and splits his time between Scotland and the US.
At his weekend getaway – in the ultraselect enclave of Isleworth, Florida – nearby residents are Tiger and basketball ace Shaq.
It was Johnston who was the architect of Woods’s $60million deal with Nike at the age of just 20.
Johnston, then IMG’s head of golf, revealed the secret of his successful negotiations saying: “Do you know how we got so much for Tiger? “Because we asked for it. We knew what he was worth to the right people.”
Johnston’s responsibilities have included representing tennis stars Serena and Venus Williams and F1 ace Michael Schumacher as well as advising both the Royal and Ancient Golf Club and Wimbledon.
But Rangers lie closest to his heart.
As he was unveiled as the new chairman after five years as a director, Johnston praised the contribution of Murray to the club.
He said: “David richly deserves his place in the Rangers history as one of the club’s greatest chairmen and his passion has burned brightly for 20 years.
“He invested heavily in the club but has also demonstrated enormous personal commitment at all times.
“On behalf of all Rangers supporters, I wish him the very best for the future.
“I consider it a great honour to be appointed chairman of the club and can say to our supporters that the board, management and staff will spare no effort in striving to ensure that Rangers Football Club enjoy a successful future.”
Johnston’s sporting hero is John Greig, the Rangers skipper of the 1960s and 70s who is now a fellow Ibrox director.
He once said said: “He was the guy who epitomised Rangers for many of the years I was following them from the early 1960s and onwards.”
He remembers all too clearly being on supporters buses heading to Ibrox from Partick to watch Greig’s side. But Johnston’s career took a remarkable turn in 1968.
He was an accountancy student at Strathclyde University when he managed to secure a summer job as a steward at the Open in Carnoustie.
The ambitious young man somehow managed to wangle a brief meeting with McCormack, whose role as an agent to the sports stars of the time was legend. And that helped launch his career.
More than 40 years on, he wields influence and power and has taken the reigns at the club he loves.
Johnston remembers the pain of growing up when the club were also-rans to a dominant Celtic side.
He once said: “Rangers fans, and I’m one of them, don’t have a high degree of patience because they’ve been used to success over the last 120 years or so.
“I have an understanding of that. I lived in Partick at the time of Celtic’s nine-in-a-row.”
Johnston has invested more than £1million of his personal cash in Rangers and helped engineer the investments of £40million made by Joe Lewis and £20million by Dave King to help during the big-spending years.
And he has been fiercely protective of his friend Murray and his reputation in the face of the minority of critics among the Rangers support.
He said: “David Murray is an extremely successful and adept businessman.
“Anybody who suggests otherwise is not looking at his record. He’s one of Scotland’s greatest entrepreneurs and the country needs a lot more David Murrays in terms of the big picture, so I reject that out of hand.
“David at no time passed on anything he wasn’t prepared to undertake himself in terms of the risk and his commitment to the club.
“The fans quite rightly look at the performance on the field because that is what is first for them but to second-guess his stewardship when he was only guilty of too much ambition on their behalf is somewhat incongruous.”
He has also endorsed the more cautious approach which Rangers have now been forced to adopt with their finances.
When he joined the board, he said: “I believe the model Rangers have introduced will allow the debt to be downscaled.
“I think they can perform as a business and as a club in the short term to regain, or retain, whichever word you want to use, their dominance in Scottish football.
“Rangers’ first agenda is to be the champions of Scotland and win every domestic trophy because if they don’t do that the next level is not accessible. Despite the present situation, Rangers are a huge football institution in the world of sport.
“They’re a terrific brand name, have a terrific following and support, and my agenda as a director is to represent the stakeholders of the club – by which I mean the fans, employees and the shareholders.
“There are a lot of different elements in terms of Rangers’ role, or potential role, in the big football picture.
“I have a fairly good idea of how various clubs exploit media rights, plus sponsorship opportunities and how to exploit branding. I’ll be able to offer some counsel in that area.
“I’m on the board as an individual with a lot of passion for the club, although my 30 years of experience in the sports world is something I’d like to think is acceptable to Rangers.”
He also sees the commercial benefits which can accrue to the Old Firm if they work together in certain areas.
Johnston said: “It is extremely important that there is the competition between Rangers and Celtic on the field. That’s what gets the juices of the fans going, and is the lifeblood of the game.
“But off the field, there is more that unites Rangers and Celtic than separates them – both are dealing in the same economy with the same circumstances and parameters.”
Apart from football, Johnston’s passion is golf. He is thought to be one of the world’s most prolific golf book collector, with a library rivalling that of the USGA.
It contains 16,000 volumes and his stated goal is to collect every golf book ever published. He began collecting in 1969 and adds 600 to 700 new books each year.
Johnston has occasionally published a bibliography of his collection.
These are privately printed books in limited editions of 10.
The bibliographies themselves have become collectable and a copy of his 1997 edition sold for $1265 at auction.
He has also written several golf books and his epic work The Chronicles of Golf regularly sells for more than $1500.