Berger has been called the "father of the American mutual fund" - mutual funds are vehicles through which small guys can invest their savings in the stock market. Berger's mutual funds tended to have spectacular growth records. He prided himself on investing heavily in his own funds so as to demonstrate that his interests coincided with the investors'. His last throw was the most spectacular: his two main funds were valued at $20m in 1989/90 but when he retired and sold out in 1994 they were worth $3bn.
A tall man - he was well over six foot - Bill Berger had white hair and a white beard in his latter days and looked like Father Christmas. The similarity was underlined when he donned a bright red caftan for dinner - which he did on the first occasion we met. He kept a 17th-century Chinese temple guardian in his hallway, almost as tall as himself, and the two of them startled his guests as he opened his own front door with a hospitable welcome.
Berger and his second wife, Bernadette, filled their home with a dense miscellany of art treasures. The mythological romp by Francois Boucher which he gave Bernadette as a wedding present set him back $574,500 at Christie's in New York in May 1996. "Boucher is corny to me, except for his drawings," Berger told me. "But I suppose he's better than a lot of other painters of chocolate-box tops and greetings cards." He and Bernadette owned a fine group of Boucher drawings.
Buying art was a shared enthusiasm. The sitting-room table was always littered with colour transparencies of art works. While they were debating a purchase, they would attach the transparency to the lampshade for easy visibility and reference. A David Roberts painting of a cathedral interior had just been unclipped when I came to dinner - because they had bought it - to be replaced by a dazzling piece of medieval English embroidery, or Opus Anglicanum.
Berger emphatically represented the best in the tradition of American capitalism. While amassing a vast fortune, he retained an innocent enjoyment of life's pleasures from food, to art, to fly fishing. He also retained a powerful idealism. He is survived by four children and three stepchildren, whom he has left a competence. But he did not believe that inherited money was good for the character. Most of his fortune is tied up in two foundations, one devoted to teaching young people to know and love nature, and the other to getting the public to love art - particularly British art.
Why British art? "Britain's where our language comes from," he told me, "where the computer comes from and - hell! - almost everything else."
Berger's grandfather was one of the co-founders of the Colorado National Bank and he had banking in the blood. He was born in Denver in 1925 and studied English literature at Yale - interrupting his studies to serve as a volunteer ambulance driver with the British Eighth Army in Italy in 1944 and 1945. He graduated in 1948.
He began managing $100m worth of equity portfolios for the trust department of his grandfather's bank in 1950 - a staggering amount of money at the time. In 1960 Berger invented the first tax-free exchange fund, the Centennial Fund, causing the Internal Revenue Service to change its rules to close the loophole he had discovered.
In 1974 Berger founded Berger Associates to manage two portfolios, the Berger 100 Fund, which invested in small companies, and the Berger 101 fund, named after another (short-lived) tax loophole. He sold Berger Associates in 1994 for $47m.
It was after retirement that he turned his attention to art. "Before that every ounce of my emotion had to go into the market," he told me. His passion for British art was born in April 1996 when he saw a portrait of Queen Elizabeth I on the cover of a Sotheby's catalogue. He and Bernadette were fascinated by the opportunity to buy into history. "With Queen Elizabeth we felt we could get a grasp of the Elizabethan era - an era full of raw emotion and intrigue." She cost them pounds 128,000 and they followed up by buying a spate of Tudor portraits, then branching out into the 18th century, buying portraits and landscapes (Reynolds, Gainsborough, Benjamin West). Their British collection ended up ranging from an anonymous Gothic Crucifixion through Holbein and van Dyck, Edward Lear and Sir Alfred Munnings, to the colourful abstracts of Sir Howard Hodgkin.
William Merriam Bart Berger, investment manager and art collector: born Denver, Colorado 3 November 1925; married 1952 Claire McMenamy (died 1989; two sons, two daughters; marriage dissolved 1976), 1996 Bernadette Johnson (two stepdaughters); died Denver 29 June 1999.