Later this year the Royal Academy in London will be showing off the cream of the most extensive collection of British art amassed outside the United Kingdom. The exhibition will include some of the greatest paintings by George Stubbs; an incomparable range of work by William Blake, from single paintings to illustrated books; and major paintings by Gainsborough, Reynolds, Constable and Turner.

The collection itself, which is permanently housed at the Yale Center for British Art in Connecticut, was created by Paul Mellon (1907-99), the billionaire son of the billionaire industrialist and financier Andrew Mellon. These two men were not only passionate collectors of art; they were also great philanthropists. The wealth of the father set up the National Gallery in Washington with a core collection of 126 paintings by the Old Masters. The wealth of the son was used to shine a spotlight on a much neglected area of art: that of the British Isles. Where did this passion for all things British come from?

Paul Mellon was brought up in a gloomy house in Pittsburgh, son to a father whom he once described as “devoid of feeling”, but from a very young age, England became almost a second home. “The family used to spend their summers here,” Brian Allen, the director of the Paul Mellon Centre for Studies in British Art in Bedford Square, tells me. This discreet, softly spoken man, who has worked for the Mellon Centre “man and boy”, describes the appeal of England to the great collector.

“Every year the family rented a house on the Thames. He developed a romantic feeling for the English countryside. What’s more, he was baptised in St George’s Chapel, Windsor.” Mellon himself would have enjoyed the fact that we are taking tea together in a handsome room overlooking an English garden in the only entirely intact Georgian square left in London.

As a child, Mellon experienced the last gasps of an idyllic Edwardian England. But his links with this country were more profound than those of a mere visitor. His mother was the daughter of a Hertfordshire brewer; his father was of Scots-Irish descent. Mellon did a first degree at Yale and then, having turned his back on the world of business, as represented by his tightly constrained father, he was allowed to go to England to do a second degree at Cambridge in the 1930s. Here his passion for all things English intensified. “Cambridge I loved,” he wrote in a speech delivered in 1963, “I loved its grey walls, its grassy quadrangles, St Mary’s bells, its busy, narrow [[ streets full of men in black gowns, King’s College Chapel and Choir and candlelight, the coal-fire smell, and walking across the quadrangle in a dressing gown in the rain to take a bath. In the winter it got dark at 3.30, and all winter the wet wind whistled straight across from the North Sea.”

Amid his aristocratic friends, he acquired a lifelong love of hunting and horse racing. He also discovered a passion for collecting – initially it was colour-plate books. After service in Europe during the war, he began to amass, with his second wife Bunny, a collection of 19th-century French paintings, which were eventually bequeathed to the National Gallery in Washington. And then, in 1959, a chance meeting with a librarian at the Royal College of Art in London led to a decision to buy British. It was timely and shrewd. British art was woefully neglected in those years, and it was consequently relatively cheap.

A little later, in England once again, he met a young man of 31, the future art dealer John Baskett, who became his first curator, and then his lifelong friend, adviser and art dealer. “I met him in Claridge’s,” Baskett tells me. “I was very nervous. He was rather shy and reserved. ‘I’ve just mixed these and I thought you’d like one,’ were his first words to me. It was a large Martini he was offering me. I worked for him in America, as his curator, and then, two years later, I returned to England. He sent me back on the Queen Mary with a case of champagne.”

Throughout the 1960s, Mellon brought quickly, shrewdly, and in quantity. His first purchase was Stubbs’s great painting of a zebra, which had been transported from the Cape of Good Hope to England aboard HMS Terpsichore in 1762, and had been presented, with all due ceremony, to Queen Charlotte. This purchase united Mellon’s passion for art and animals – and he continued to buy sporting art throughout his life. “It all happened so quickly,” says Baskett. “Within three years he had bought enough to have a major museum show in Virginia, which travelled on to London’s Royal Academy, in 1964.” He also provided the funds to publish books about some of those many underrated British artists to which he was drawing attention by purchasing their works from the saleroom.

“Yes, and that publishing programme goes on to this day, in conjunction with Yale University Press,” Allen says. Getting up from his sofa and crossing to a handsome floor-to-ceiling bookcase, he proudly runs his finger along the spines of books devoted to Blake, Gainsborough, Romney and many others, and all funded by Paul Mellon’s money, wisely reinvested during his continuing afterlife – as are numerous fellowships and scholarships and much else, and all with a view to increasing an understanding of the importance of British art.

“The man was an insatiable collector, of art and books, from first to last,” says Baskett. Mellon would have agreed. “Some years ago I walked past a shop in New York called The Incurable Collector,” he once wrote, “and I remember thinking at the time that the title could equally well be applied to me.”

By the middle of the 1960s, Mellon knew that he wanted to create a permanent home for his ever-growing collection. He offered it to his old alma mater, Yale University, and a purpose-built building to go with it. With this new institution in mind came new responsibilities.

“Now he felt he had to diversify,” Allen tells me, “away from his greatest love, which was really small and intimate pictures of English country life. At this point he added grander paintings to fill in the gaps, portraits of the kind that his father, or one or another of the other plutocrats who bought from Duveen, might have bought – by the likes of Gainsborough, for example.

“And yet,” he says, “I think that his greatest passion from first to last remained horses – even more so than art.” Allen gives a wry smile. “You know, I think that the most thrilling moment of his life was when he won the Derby with Mill Reef in 1971.” Allen’s eyes are shining. He is thinking back to something which feels as fresh and as vital as yesterday. The same could be said of this great collection.

Paul Mellon’s Legacy: A Passion for British Art, Royal Academy, London W1 (www.royalacademy.org.uk 020-7300 8000), Oct 20 to Jan 27

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