From just 2,000 paintings and 50,000 prints...

Spoilt for choice. That's how the curators at the Royal Academy of Arts must have felt selecting the works to be shown at their latest exhibition, which marks the centenary of the birth of the millionaire, philanthropist and British art collector, Paul Mellon (1907-1999).

For they had to pick 150 works from the largest collection of British art outside the UK - Mellon's collection of 50,000 prints and drawings, 2,000 paintings, several hundred pieces of sculpture, and 35,000 rare books and manuscripts now in the Yale Center for British Art, Connecticut.

They made a stunning selection, however. An American's passion for British art: Paul Mellon's Legacy, open until January 27, combines some of the finest works of British art with key elements of Mellon's personal taste. It features work from the 15th to the early 20th century, including some objects not seen by the British public since they were purchased, and work by major artists, some represented several times over. There are eight Stubbs, for example, six Constables, eight Turners, four Gainsboroughs - three landscapes, one family portrait - eight works attributed to William Blake, and five Rowlandson watercolours.

Mellon was born in 1907, the son of an American banker of Irish and Scottish descent who was ranked alongside John D. Rockefeller and Henry Ford as one of the wealthiest men in early 20th-century America. Mellon spent his formative years here with his English mother, the daughter of a Hertfordshire brewer, holidaying in the Thames Valley, studying at Cambridge in the late 1920s (taking his polo ponies with him, as you do), and generally developing his passion for horses, fox-hunting, horse-breeding and racing, history and literature: in short, for all things English.

His love for British art started with horses, reflected in the opening room's Sporting Art images. But he was in middle age before he began collecting seriously, first falling in love with it (and taking his first steps towards amassing a collection of a genre in decline) as a result of meeting a gallery owner out hunting in the Cotswolds who told him of an oil painting for sale.

Mellon purchased Pumpkin with a Stable-lad (1774) in 1936, ". . . bowled over by the charming horse, the young boy in a cherry-coloured jacket, and the beautiful landscape background". It's a typical George Stubbs with its lithe racehorse, perfectly studied, and its serene country estate scenery, and remained Mellon's favourite piece of British art.

Pumpkin is in the exhibition alongside other racing images, comparative anatomy studies by Stubbs and his famous Zebra. No-one knows why Stubbs painted the zebra, the first 'African She-Ass' seen in Britain, a gift to the young Queen Charlotte in 1762, but the oil stayed in his studio until he died.

Then, in the next couple of rooms comes a superb display of landscape art, including Constable's poignant rendering of Hadleigh Castle, and Turner's Staffa, Fingal's Cave with its little black steamboat battling the forces of nature, a painting that sums up Turner's passion for wild places. In the UK for the first time since it was purchased 40 years ago, Turner's masterpiece, Dort or Dordrecht: The Dort Packet-Boat from Rotterdam Becalmed (1818), the daily ferry boat crammed with passengers, its limp flag telling a tale; done to show he could surpass Dutch art, they say. Also another Turner masterpiece, Mer de Glace, a watercolour painted with dry rough brushstrokes in 1802 and full of scraping out and stopping out.

Here too is Canaletto's Windsor Castle. Canaletto is one of a few non-Brits in the show; British art meant British-ness to Mellon, not simply nationality.

Richard Wilson's In Rome from the Villa Madonna was one of his earliest paintings, produced in 1753 as a souvenir for wealthy tourists on the Grand Tour. With its wonderful light and perspective and a tonality based on Claude, it is one of his finest, according to art historian John Basket, who helped Mellon buy it. Here too Cozens, father and son, with Alexander's dramatic Mountainous Landscape and his son John Robert's watercolour of one of the most popular stops on the Grand Tour, The Lake of Albano and Castel Gandolpho (the painting was one of Cozens' best sellers; about a dozen exist), and Paul Sandby's view of Roslin Castle (c1780), yet another picture redolent of tourism and the 'Picturesque'.

Francis Towne's surprisingly modern-looking watercolour Ambleside (1786) tells us something about Mellon's tastes. He bought it, he said, because it stirred an emotional response in him; by his own admission his collecting was based on instinct rather than intellect. His tastes also ran to smaller works, such as the Constable studies, Clouds, for instance, and the smaller portraits later in the exhibition.

William Turner of Oxford gets in with his simple, if inaccurate, view of Donati's Comet (it apparently should have twin tails). Its original title tells us it was seen Near Oxford - Half-past 7 o'clock p.m. Oct.5. 1858?.

And so on to the many manuscripts, maps, sketchbooks and books. Marvellous works all, like the parchment map of Drake's circumnavigation of the globe (c1587), Ptolemy's map of the British Isles inside a 1513 atlas, and the Helmingham Herbal and Bestiary (c1500) open on pages of English plants, and the books of hints on taste and watercolour technique by Gilpin and others.

Mellon was a book collector long before he began to collect pictures. He was interested in literature, bought beautifully bound copies of the great poets, and was "very aware of connections between art and literature", says Yale curator Elisabeth Fairman. So we have here a second edition of Chaucer by William Caxton, and another rare edition produced some 500 years later by William Morris at the Kelmscott Press (in 1896), one of two Mellon bought in 1939. Open at the frontispiece and title page it shows Morris's distinctive design of lush vines and interwoven leaves opposite an Edward Burne-Jones illustration.

Finally, if you want to see the man himself, go into the Fine Rooms on the way out where there is a portrait of Mellon astride his hunter in classic English countryside.

An exhibition celebrating the 250th anniversary of Thomas Rowlandson, known for satirising university life, coincides with the RA exhibition. It is at Lowell Libson Ltd, 3 Clifford Street, London, until November 22. Website: www.lowell-libson.com