Hilary Spurling's Matisse the Master has won the £25,000 Whitbread Book of the Year award, beating the bookmakers' favourite, The Accidental by Ali Smith.
Spurling, who spent 15 years writing and researching her two-part biography of the French artist, was chosen over the four other Whitbread winners.
She said she was "gobsmacked", adding: "My money was placed elsewhere."
Author Michael Morpurgo, who chaired the judging panel, said the biography was "an extraordinary achievement".
"It has opened our eyes to great art, and done it in an extraordinary way."
The winner of the annual prize was revealed at a ceremony at The Brewery in London on Tuesday evening, where Spurling picked up her cheque.
"We all agreed when you get to the end, you are sorry it is finished - an extraordinary achievement for a book of this length," Morpurgo said.
Spurling said: "If Matisse knew the way that this award went tonight he would see it as another tribute from a country that responded to him and loved him always from the beginning."
Matisse the Master is the fifth biography to take the overall Whitbread prize. Claire Tomalin was the last biographer to take the prize in 2002 for Samuel Pepys: The Unequalled Self.
Spurling, 65, faced competition from Smith, first-time novelist Tash Aw, poet Christopher Logue and children's author Kate Thompson.
Each of the five won £5,000 in separate Whitbread award categories earlier this month.
London-based Spurling, a former journalist for The Spectator, regularly reviews books for the Observer, The Daily Telegraph and the New York Times.
The author began work on the first volume of her Matisse biography, The Unknown Matisse, in 1990. Spanning the early years of the French painter's life, it was published in 1998.
Spurling was given unprecedented access to the artist's family papers to research the detailed and illuminating biography, prompting Whitbread judges to hail the second volume "a masterpiece" earlier this month.
She dedicated the award to Matisse's heirs, his grandchildren, who showed "great generosity and courage" in giving her access to his papers.
The author said she was told Matisse would be "too dull to write about".
"I could not believe that someone who painted such pictures could have so dull a life. I was right. Matisse was a drama-a-day man, he made Picasso's life look like a picnic," she said.
Ali Smith's The Accidental had been favourite to win the £25,000 prize
Spurling added she had expected it would take seven years at most to write the book, but it had been a labour of love.
"I never lost faith in Matisse, I mean he was endlessly, has been endlessly surprising, and still is surprising," she said.
Author Andrea Levy won last year's Whitbread Book of the Year award for her novel Small Island, which went on to become a major best-seller.
The awards, which attracted a record 476 entries this year, were established in 1971 and are open to authors based in the UK or Ireland.