Sir Denis Mahon: Art collector who fought for free admission charges and against the sale of works from public collections

Following the recent death of the Duke of Grafton we have lost, with the death of Denis Mahon, the last of that generation of men of independent means who dedicated their lives to the public good. Grandson of the 5th Marquess of Sligo and a beneficiary of the Guinness Mahon merchant bank, Denis Mahon was educated at Eton and Christ Church, Oxford, where he read history and took tutorials from Kenneth Clark, who in turn introduced him to Nikolaus Pevsner, then teaching at the Courtauld Institute. Pevsner had previously researched Milanese Seicento (17th century) painting and he suggested that Mahon should study the neglected Bolognese painter Giovanni Francesco Barbieri, Il Guercino.

The publication by the Warburg Institute in London of Mahon's Studies in Seicento Art and Theory in 1947 marked a watershed in the study and appreciation of 17th century Italian painting, and above all the Bolognese painters and Guercino. Since the late 1930s Mahon had been studying the source material and making occasional purchases, including The Coronation of the Virgin by Annibale Carracci, painted in 1603 for Cardinal Pietro Aldobrandini. As a scholar and connoisseur of the old school, Mahon combined a huge knowledge of the documentary sources with meticulous visual analysis, both of which he deployed to devastating effect.

Many art historians of renown lack an "eye", and no amount of documentation and accretions of critical literature can compensate for the lack of "gut reaction" to any individual work of art per se. That instinctive response, combined with modest financial resources when prices for Seicento paintings were rock-bottom before the 1960s, made possible the acquisition of one of the finest private collections of its kind in the world, exhibited at the National Gallery in 1997 as "Discovering the Italian Baroque". Speculation about the future of the Denis Mahon Collection reached new heights.

Throughout his life Mahon was bitterly opposed to the sale of art from public collections (deaccessioning) and to charging for entry to public museums and art galleries which housed them. Both concerns were to provide vital threads through his life, and the Bill intended to permit the Trustees of the National Gallery to sell pictures which had fallen out of favour enraged him. Blessed with private means, he could not be silenced by politicians and bureaucrats used to bullying public servants, and few were surprised when the Prime Minister appointed him a Trustee of the National Gallery (1957-1964, and again 1966-1973).

One of his first successes as such was to persuade his fellow Trustees to purchase the huge Adoration of the Shepherds, by Guido Reni, from the Prince of Liechtenstein in 1957. With great relish he would tell the story of how it was too large for him to study in Schloss Vaduz and that when two stalwart lederhosen-clad retainers took it out into the meadow, the wind caught it and the painting descended into the valley below with the retainers hanging on to it.

In art-historical circles another watershed was reached in consequence of the great 1960 Poussin exhibition mounted in the Louvre under the baton of Sir Anthony Blunt. This put forward a chronological development of the painter which was not universally accepted. Mahon was never willing to bury such differences merely out of deference to the Director of the Courtauld Institute, and, although not well known for his interest in Nicolas Poussin, put forward an alternative sequence based on rigorous visual analysis undertaken in the presence of the paintings and old-fashioned connoisseurship. This he put forward in a series of articles and it has become generally preferred to that of Blunt.

The Blunt-Mahon controversies took on an acid tone, and when I, as a student at the Courtauld Institute, had the temerity to spend an afternoon with Mahon at 33 Cadogan Square in 1962 I was carpeted by the irate Director. However, it was the beginning of a long and close friendship for which I am eternally grateful. This was reinforced in Bologna that autumn when Mahon and Michael Kitson were setting up the "L'Ideale Classico del Seicento" exhibition there. Subsequently we travelled together to exhibitions in Italy and Germany.

Blunt joked to his students that he no longer had to bother to correct his proofs because Mahon did it for him, and relations were not exactly improved in 1964 by the latter's recognition of a painting at Sotheby's, attributed there to Pietro Testa, as an important missing Poussin which had been painted for Cassiano dal Pozzo. Indeed, Mahon's election to the British Academy that year was opposed – unsuccessfully – by Blunt. On the other hand, Mahon did not treat this academic accolade over-seriously, observing to me, half in jest, that FBA after his name ensured that The Times published rather more of his letters! Public recognition of his dedication came in 1967 when he was appointed CBE.

The year 1968 saw the great Guercino Exhibition in Bologna and, in retrospect, the most hair-raising of our expeditions. The Italian Air Force provided a Lockheed Northstar paratroop aircraft for the transport of the majority of the British loans, escorted by Mahon, Andrew Wilton (then of the British Museum), Henry Gerson and myself. The first problem was that the runway at RAF Northolt was too short and the flight had to be transferred to RAF Benson with the aircraft partially loaded. Since the flight was under Nato regulations each escort had to wear two parachutes. Then the Italians announced that we had insufficient fuel to reach Rimini and we would have to put down at Marseilles.

This meant a fresh flight plan and the diverted flight ran into a violent thunderstorm over the Massif Central and some six inches were burnt off the Pitot tube by a lightning strike. Mercifully, we were not told until we had landed in Rimini that we had had no means of measuring our airspeed. The aircraft contained 16 major paintings by Guercino and the cream of the Guercino drawings from the British Museum and Mahon's own collection.

Returned to power in 1970, under Edward Heath, the new Conservative government was soon the target off Mahon's lobbying when David Eccles, as Paymaster-General with Responsibility for the Arts, espoused the policy of charging for museum entry. Visitors to 33 Cadogan Square many years later will recall the stacks of yellowing papers up the stairs labelled "DM v. Eccles", while the mountain of copies of Hansard totally encasing the sideboard was a feature of the dining room.

His mother, Lady Alice Mahon, died in 1970, and Mahon attempted to pay the capital taxes on her estate by offering to the nation The Coronation of the Virgin by Annibale Carracci, from his collection. This painting was worth substantially more than the taxes payable, but HM Treasury declined to credit him with the difference. He sold it to the Metropolitan Museum, New York, in 1971, demonstrating that he had not been bluffing.

The mid-1970s saw a series of major exhibitions focussed on the 17th and 18th centuries, and in preparation for the "Lombard Paintings" exhibition in Birmingham in 1974, Mahon and I spent many days in the huge "Il Seicento Lombardo" exhibition in Milan (1973) attempting to clarify chronologies and visiting the Sacri Monti of Lombardy and Piedmont. These sanctuaries composed of chapels containing life-size, highly realistic sculptures with glass eyes and horse-hair wigs acting out scenes from the Bible and the lives of the saints, reached their climax in the Seicento. Mahon was deeply moved by them and remarked that he subsequently looked at Seicento religious art with different eyes .

Also mounted in 1974, in Florence, was the "Ultimi Medici" exhibition of Florentine art around 1700 and the accompanying Symposium there. Mahon lent to it his huge modelli by Luca Giordano for the Palazzo Medici Riccardi in Florence, but their status was questioned in unflattering terms by an Austrian art historian. A detailed analysis of the differences between the modelli and the finished ceiling paintings enabled the working practices of Luca Giordano to be reconstructed, and visitors to the Palazzo the following day were entertained to find Mahon and me lying on our backs with binoculars looking for the tell-tale pinholes in the ceiling paintings. This work I published in The Burlington Magazine in December 1974.

In a wonderfully generous gesture of mutual support, when the campaign was mounted in 1977 to purchase for the City Museum and Art Gallery of Birmingham the Cornbury Park Bellini, Mahon sold his Poussin in order to be able to make the biggest single contribution from an individual.

Working with Mahon was always a stimulating experience and he was immensely thoughtful in many ways. He was instrumental in the founding of Heritage in Danger, in 1974, which brought together with him Hugh Leggatt, Andrew Faulds and Ernle Money, while an outer circle of friends busied themselves with fanning the flames of controversy. Sir Edward Muir, formerly Permanent Secretary of the Ministry of Works, once said to me, "You are a friend of Mr Mahon, cannot you persuade him at least to be polite to us?" Never a lover of bureaucrats and politicians he perceived to be unsympathetic to his concerns, Mahon was ruthless in using the future of his collection as a means of persuading successive governments to adopt his point of view. It is said that changing one's will is the last blood sport left to the elderly, and Mahon kept everyone guessing almost to the end.

During decades of his life Denis Mahon continued to be devoted to the causes he supported, but compared to the hurly-burly of the 1960s and 1970s he increasingly preferred to work behind the scenes.

Peter Cannon-Brookes

On Tuesday and Thursday mornings, from 10.30am-1pm in the spring and summer of 1980, the National Heritage Bill trundled its way through the committee stage in Committee Room 12 of the House of Commons, writes Tam Dalyell. For the first two mornings, an elderly, somewhat ramshackle, large man, shuffled into one of the seats available to the public. On the second session of the committee I saw him shaking his head during a minister's answer to a question I had asked. Curious, I went across to him when the proceedings adjourned, and asked him why he had indicated dissent. His reasons were erudite. Would he care for a spot of lunch to continue the conversation? He accepted. Down we went in the lift to the Members' cafeteria to queue, for a dollop of stew and potatoes.

The next morning my friend, sparring partner and opponent, Alan Clark, MP for Plymouth Sutton and future diarist extraordinary, buttonholed me in the library. "I am told, Tam, you took Denis Mahon to the cafeteria yesterday!" "Well, Alan," I replied, "I did take an old boy who came to the Heritage Bill Committee. He seemed to know one heck of a lot about the clauses of the bill. I didn't like to ask him his name."

"But," smirked Clark, "Denis Mahon has never eaten a meal in his life before, in a cafeteria." He explained that Mahon had been his father's favourite student, when Sir Kenneth Clark, he of Civilisation, was Keeper of the Department of Fine Art at the Ashmolean Museum. Alan informed me that Mahon had the greatest collection of Italian 17th century paintings in private hands in Europe, worth many millions. So the next day I invited Mahon again – to the more salubrious Strangers Dining Room, which initiated a long friendship.

In truth, Mahon drove to distraction junior ministers responsible for taking heritage bills through parliament, and civil servants – particularly Treasury civil servants – because he had a direct line to a number of cabinet ministers, and did not feel inhibited about threatening not to give his collection to the nation unless they accepted some of his views. Mahon can claim powerful credit for winning the battle against museum charges, and most of the credit for the great "acceptance in lieu" device, under which works of art can be given to the nation to offset death duties without cash limit.

John Denis Mahon, art historian, collector and lobbyist: born London 8 November 1910; Trustee of the National Gallery 1957-64 and 1966-73; Member, Advisory Panel, National Art-Collections Fund, from 1975; CBE 1967, Kt 1986, CH 2003; died 24 April 2011.