He and his books are firmly established in Pepper Pike, an affluent suburb of Cleveland, Ohio, but his fervent wish is to find a home for them in Scotland. "That's the plan," he says. "It's about the heritage of our country after all. As long as I am relatively active, I will continue to build it here, but I would like to make arrangements before I have gone. I want it to be in Scotland, and I want to provide for it financially. The people should be able to enjoy it, have access to it, look after and add to it."
Johnston returns to the country of his birth at least 10 times a year, as he has done for this week's Open Championship at Turnberry. He could have made a fortune already from the sale of his collection, but the hobby that started by accident, quickly became a passion and now consumes his leisure time still gives him enough enjoyment to make its value an irrelevance.
He is not prepared to put a price on it. "Across here, developers are building unique golf resorts all over the place, and I get some of them asking how much I want for the library. They would like to relocate it in their clubhouse, but in all honesty, I'm not interested. As long as I can enjoy shuffling around in there, it will be staying where it is. It's my way of relaxing. Arnold Palmer regrips his clubs; I potter around in my library."
Palmer is one of the reasons Johnston has been able to pursue his passion. The two have been close since 1977, when IMG founder Mark McCormack made him the player's business manager.
The Scottish lawyer who grew up on Glasgow's south side had famously asked McCormack for a job while stewarding in the 1968 Open at Carnoustie. After a brief internship in America, he returned with an armful of books on the world's greatest golfers.
Johnston was hooked. Within a few years, the interest had become a fascination, and later still an obsession, which he fed on business trips with IMG. "When I first took it really seriously, over 25 years ago, it was a case of going into a town, looking up the yellow pages and hunting down the antiquarian bookshops. I would go out for a jog, arrive all sweaty and disgusting with a satchel on my back, and return to my hotel with it full of books."
Johnston also has a home in Isleworth, Florida, where he is a neighbour, and business associate, of Tiger Woods. Once described as among the most powerful men in sport, the Scot knows plenty who dismiss his library as a trivial pursuit. "I have created something that is the best in the world, and I have friends who come from all over to see it, but there are others who couldn't give a damn. When they come to my house, they go straight to the bar."
Johnston acquires between 500 and 800 books every year. He claims to have more of them than the United States Golf Association, whose library in New Jersey uses his as "a marker". As well as books, he has everything from tournament programmes to instructional pamphlets and club histories. The extension, which he describes as consistent with the architecture of his contemporary home, has been designed in such a way as to maximise security. Given that natural light does the most damage, windows have been positioned accordingly, while glass cases protect the most valuable pieces from dust.
Rare artefacts include a letter written in 1680 by King James VI in which he extols the pleasures of golf to his niece, the Countess of Lichfield. He also has a copy of the 1457 Scottish Acts of Parliament, which provided the first mention of golf in print. One of his greatest finds is an original copy of The Goff, the first book devoted to golf, written by Thomas Mathison in 1743. "And it is an excellent copy," says Johnston. "A first edition. I only know of five or six first editions in the world, and mine is a very good copy, better than the British Museum's by the way."
He doesn't say what he paid for The Goff, but someone is reputed to have offered $100,000 for it in 2003. With even his bibliographies fetching four-figure sums at auction, Johnston could make a fortune from one-off sales, but he has no intention of doing so.
"I am in the fortunate position of having no reason to sell. I don't accumulate for the purpose of resale. It would be impossible for anyone to build this collection again, and my friends have prevailed on me not to break it up. It is the magnitude of it when congregated that makes it unique."
The book that means more to Johnston than any other is The Chronicles Of Golf, not because it covers every mention of the sport between 1457 and the mid-19th century, or because it amounts to an exhaustive social history, but because he wrote it himself, with the help of his late father, James. Together, they spent eight years writing and researching, without pay, a document that effectively cost Johnston junior his marriage.
His father, who had just retired, did most of the "donkey work", travelling back and forth from Glasgow to Edinburgh, where archivists helped him to trawl the vaults. His son had briefed him to follow the recommendation written by Scots poet Andrew Lang in 1890. It read: "A young man must do (the study] and he will be so ancient before he finishes the toil that he will scarce see the flag on the short hole at St Andrews from the tee."
Well, the Johnstons did all that and more, even if they weren't sure why. "I wondered that 1,000 times, but once I had decided to do it, I had to do it all. It has become a cult item for the people who care. I did several hundred in a limited edition, and they are still selling for $2,000-3.000 a time. I am immensely proud of it."