An eminent historian and collector of Italian baroque art, he was also a resolutely effective lobbyist for free museums
Sir Denis Mahon, who has died aged 100, was one of the most distinguished art historians and collectors of the 20th century, and a determined campaigner on behalf of museums into his 90s. His collection of Italian baroque paintings, including masterpieces by Guercino, Guido Reni and Luca Giordano, has been deposited in British institutions, including the National Gallery in London, the Ashmolean Museum in Oxford, the Fitzwilliam Museum in Cambridge and the National Gallery of Scotland in Edinburgh.
Twice a trustee of the National Gallery (1957-64, 1966-73), he was instrumental in pushing through several important acquisitions, including Reni's Adoration of the Shepherds, still the largest painting in the gallery, and, in 1970 (with the much-appreciated support of the sculptor Henry Moore, a fellow trustee), Caravaggio's Salome Receives the Head of St John the Baptist, a very moving late work by the artist. He was a zealous guardian of the public interest, actively opposing the imposition of charges for visitors to museums and lobbying for legislation to prevent the National Gallery from selling any of its pictures.
Together with the National Art Collections Fund (now the Art Fund) and others, he pressed for the conversion of the Land Fund – set up in 1946 as a memorial to those who had given their lives for their country and which had then been conveniently forgotten, as he put it, by the Treasury – into the National Heritage Memorial Fund, with independent trustees. His collection was always a very effective weapon in these campaigns, and he several times threatened to dispose of it abroad if the government of the day, Labour or Conservative, failed to live up to its responsiblities in defence of museums and heritage.
His natural attitude towards ministers and bureaucrats was one of suspicion, and theirs towards him was one of respectful fear combined with intense exasperation. He did, however, have a good working relationship with several arts ministers, including the Conservative Grey Gowrie (1983-85) and the Labour culture secretary Chris Smith (1997-2001).
Even after making public the details of his decision in 1999 to bequeath his pictures to the national collections fund, with the instruction that they be deposited in various British galleries and museums, he still threatened to change his arrangements if the government refused to change the rule whereby the non-charging national museums – including the British Museum, National Gallery and National Portrait Gallery – could not reclaim VAT, whereas the charging museums – the Victoria and Albert, Natural History and Science Museums – could. In 2001 the government agreed to remove the anomaly, a gesture that gave Mahon real pleasure.
Born in London, he was the the son of John FitzGerald Mahon, a member of the family that had prospered from the Guinness Mahon merchant bank, and the grandson, through his mother, Lady Alice Evelyn Browne, of the fifth Marquess of Sligo. On visits with me to Kenwood House, on Hampstead Heath, north London, he would point out with evident satisfaction that the Portrait of Countess Howe, one of Thomas Gainsborough's finest paintings, showed his great-great-great-grandmother.
Educated at Eton and Christ Church, Oxford, where he read history, Mahon developed a great enthusiasm for opera. A career in the family business held little attraction for him, and he soon decided to devote himself wholly, and to the exclusion of the pleasures of opera, to studying the history of art. Kenneth Clark, then at the Ashmolean, was an influential figure in his formative years and recommended him to Nikolaus Pevsner, the German émigré art historian, who was then teaching at the fledgling Courtauld Institute of Art in London.
Best known today for his writings on the architectural history of Britain, Pevsner had early been interested in Italian baroque art, and suggested to Mahon that he study the work of Guercino – the little squinter – the nickname of Giovanni Francesco Barbieri, a neglected Bolognese painter of the 17th century. Guercino was an ideal subject because his life is well documented, and he was remarkably well represented in British collections. The largest group of drawings by him is at Windsor Castle, in the Royal Collection, which Mahon was to catalogue, together with Nicholas Turner, in 1989.
In his 20s, Mahon travelled extensively to study the works in museums and private collections of the Bolognese painters: the Carraccis – the brothers Annibale and Agostino, and their cousin Ludovico, who were jointly responsible for a revival of Italian painting at the end of the 16th century – Reni and, of course, Guercino, who remained his principal interest throughout his life.
Together with the Viennese expert Otto Kurz, another member of the diaspora of Jewish art historians who had left Germany and Austria in the 1930s, he visited Stalinist Russia, with a trunk of full of antiquarian books, including the biographies of artists written in the 17th century by Gian Pietro Bellori and Carlo Cesare Malvasia. The customs officials demanded that the English newsprint that had been used to wrap up the books be removed and handed in to the authorities, and informed them that they could claim it back when they left.
Mahon's studies were intimately bound up with the formation of his own collection of paintings and drawings. In 1934 he bought his first picture by Guercino, the large Jacob Blessing the Sons of Joseph, now in the National Gallery of Ireland. Two years later, he bought for £200 Guercino's Elijah Fed By Ravens, another early work, from the Barberini collection in Rome. At this time he was serving as honorary attache – an informal, unpaid curator – at the National Gallery, then under the directorship of Clark, and he offered to sell the painting at cost price to the gallery. Clark thought very highly of the painting, but felt that it would be impossible to persuade the trustees of the merits of buying an Italian baroque painting.
That gave Mahon the idea of forming a collection, with a view to holding them until such time as the national collections would be interested in having them. It is difficult to appreciate now how despised Italian seicento – 17th-century – pictures were in Britain in the late 19th and early 20th centuries. This disdain was partly due to the influential views of John Ruskin, who detested the Carraccis, referring to them as "scum of Titian", and the overwhelming preference among collectors and museum curators for the "Italian primitives" of the 13th to 15th centuries.
Mahon, unaffected either by the prevailing view or by religious prejudice, acquired – for fairly small sums – paintings of intensely Catholic subject matter by the then unfashionable painters Domenichino, Andrea Sacchi and Ciro Ferri. In 1945 he encountered Reni's The Rape of Europa, a late masterpiece by the painter commissioned by the king of Poland. At the auction he had found himself bidding against the frame dealer Arnold Wiggins, who was only interested in the beautiful régence – early 18th-century – French-carved gilt frame that was on the picture. Both painting and frame are now in the National Gallery.
In 1947 Mahon published his Studies in Seicento Art and Theory, which remains a fundamental text on 17th-century Italian art. It contains a series of closely argued essays, analysing, among other things, Guercino's change of style over the course of the 1620s, the significance of the theoretical writings of Giovanni Battista Agucchi, and a devastatingly thorough refutation of the label "eclectic", which had been used to describe the Carraccis since the 18th century. The book was very significant in winning over the institutions of art history in Britain to the acceptance of Italian 17th-century painting as a major and respectable school of art.
In the 1950s and 60s Mahon turned his attention to Caravaggio, writing some very important articles and identifying pictures previously thought lost, including the Young St John the Baptist, which he saw hanging in the office of the mayor of Rome, where it was considered an old copy. It is now one of the masterpieces of the city's Capitoline Gallery. Roberto Longhi, the leading Italian figure in the study of Caravaggio in the 20th century, was clearly irritated by Mahon's incursions into what he considered his territory, but had to recognise that Mahon's observations were usually right.
In the 1960s some of Mahon's finest writings were on the French classicist painter Nicolas Poussin, and there he often found himself at odds with Anthony Blunt, surveyor of the Queen's pictures, director of the Courtauld Institute and the principal authority on the artist. Mahon was critical of Blunt's connoisseurship and his confused reconstruction of the chronology of Poussin's oeuvre. Once again, Mahon's views have generally proved to be right, and his Plea for Poussin As a Painter (1965), an appeal to interpret the artist less as a peintre-philosophe and more as a pure painter, has been widely heeded.
In 1964 he bought at Sotheby's an early work by Poussin, Rebekah Quenching the Thirst of Eliezer at the Well, which in the sale catalogue had been misattributed to the Roman painter Pietro Testa. Astonishingly, in 1995 – by which time Mahon had long stopped collecting – the story was repeated when he identified, again at Sotheby's, Poussin's The Sack of Jerusalem, also misattributed to Testa. The painting now has pride of place in the Israel Museum.
With Cesare Gnudi, the soprintendente of Bologna's art institutions, he worked on a series of groundbreaking exhibitions in the 1950s and 60s: the Mostra (exhibition) dei Carracci, the Ideale Classico del Seicento in Italia, and, in 1968, Guercino. Mahon was always a generous lender to exhibitions himself, and in 1960 much of his collection was shown at the vast Royal Academy exhibition Italian Art and Britain. In 1997 his entire collection of nearly 80 paintings was shown at the National Gallery, together with a selection of his drawings by Guercino that he had deposited years earlier in the Ashmolean Museum, and two paintings by Annibale Carracci and Guercino . After this, the collection never returned to his home in Cadogan Square, Knightsbridge, central London, where he lived with his Spanish housekeeper and her family, since he feared that all the publicity it had received put it at risk from thieves.
Though he was never formally a teacher, he taught a great deal to many academics, university students, museum professionals and dealers through conversation, in exhibitions and in front of pictures. Many visitors to the great Poussin retrospective at the Royal Academy, London, in 1994-95 will have noticed an elderly gentleman in a slightly baggy pinstripe suit holding a stick that he would wave perilously close to the pictures, explaining the intricacies of Poussin's early development, or the dating of the late landscapes to anyone who might be interested. He spoke about painting in a brilliant and illuminating way, and his ability to sympathise with the aims of the artists he most loved meant that he spoke of Poussin, Guercino, and Annibale Carracci as though they were people he knew.
He celebrated his 100th birthday at the National Gallery last year, in the Italian baroque gallery that he had played such an important role in shaping, in the company of many friends and admirers. He was knighted in 1986, and appointed Companion of Honour in 2003.
• John Denis Mahon, art historian, collector and philanthropist, born 8 November 1910; died 24 April 2011