Edwin Manton, who died on Saturday at age 96, was the president and chairman of the insurance company AIG, and, later in life, one of the most prolific benefactors of the Tate Gallery, the British art museum.

An avid collector of Constable for over half a century, he owned so many paintings and drawings by the artist that, when seven disappeared from his New York apartment in 2000, he did not immediately notice. More extraordinary still, none of the works was insured. (It later turned out that the works were merely misplaced during a renovation.)

Nearly unknown in Britain, where he retained his citizenship despite living in New York since the early 1930s, he came to public attention in his home country after donating L12 million to the Tate Gallery – one of the largest private donations it had ever received.

Manton’s connection with the Tate dated back to the 1980s, and in 1987, he was one of several anonymous donors who helped raise L3 million to save Constable’s “Waterloo Bridge” from being exported from Britain. In 1992, he gave the gallery L7 million towards a L31 million redevelopment of its new Tate Gallery of British Art.

At the time, he insisted that his identity remain confidential: “I made my gifts anonymously to protect myself from people importuning me. It was not a noble feeling. I was simply protecting my purse.” His identity was disclosed in 1994 when he was knighted for charitable services to the gallery.

In 1997, he announced that he would be giving a further L5 million as a mark of his “deep gratitude” to Leslie Parris, deputy keeper of the Tate’s British Collection, who had advised him that a Constable he had wanted to buy was, in fact, a fake. At the same time, he announced his intention to leave to the gallery Constable’s “Glebe Farm,” a previously unknown painting that had surfaced in America two years previously.

Edwin Alfred Grenville Manton was born on January 22, 1909, at Earls Colne, Essex, less than 20 miles from Constable country on the Suffolk/Essex border. He was known ever after as Jim and then as Sir Jimmy thanks to his resemblance to Sunny Jim, the energetic logo character for Force wheat flakes. He won a scholarship to Cambridge, but never took it up; instead he opted for a job in insurance, which took him to the Paris office of Caledonian Insurance.

Five years later, in 1933, he set sail for New York, where he started work as a casualty underwriter for the American International Underwriters’ Corporation. Manton rose rapidly through the ranks to become president in 1942, a post he held for the next 27 years; he took over the chairmanship in 1969. He retired officially in 1975, but returned as a senior investment adviser to the reconstituted American International Group in 1982.

The company employed only 13 people at the time he began working for it, but by the 1990s there were 53,000 on the staff. Manton derived his L260 million fortune from the 4 million shares he owned in the company. “None of this was planned, it just happened,” he once confessed.

Manton bought his first Constable in 1945, but was dismayed to discover that it was a fake, painted by a German artist. By his own admission, his collecting remained a hit-or-miss affair, and he later reckoned that about half the works he had bought on the assumption that they were by Constable had turned out to be by other artists. He could not explain why he had fallen for Constable, but admitted, “I am a compulsive buyer.” After all, he observed: “It’s better than spending your money on bottles of Scotch.”

To avoid taxes, Manton planned to make over his remaining Constable collection to a charitable foundation that would loan the works to public collections, allowing his grandchildren to “see what grandpa spent his money on.” He described the American taxman as “the big enemy” who, given half a chance, would confiscate his wealth on his death.

In 1997,Manton traveled to London to be introduced to the Prince of Wales at the Tate’s centenary celebrations; in a speech, the prince paid tribute to Manton’s great generosity to the gallery.

Manton belonged to several clubs in New York, including the St. George’s Society for wealthy expatriates; he was also a trustee of St. Luke’s Roosevelt Hospital. In 1998, he was elected to the American Insurance Hall of Fame.

He listed his hobbies as “walking and art,” and – in deference to his advancing years – “formerly cricket and [field] hockey.”

Surviving are his daughter, Diana Morton, two granddaughters, and five great-grandchildren.