Spurling, 65, came out on top for Matisse the Master, the second volume of a biography she penned about the great painter.
The Whitbread's top honour goes to one of the winners of prizes already awarded in five categories - novel, first novel, poetry, biography and children's book.
Each category winner receives about three thousand pounds (A$6,600), while Spurling receives the 25,000 pound (A$59,000) Whitbread Book of the Year Award.
Born in Stockport, northwest England, Spurling spent 15 years writing the two-volume tome. The Oxford graduate had unprecedented and unrestricted access to voluminous family correspondence and other new material in private archives.
"I hope if Matisse could have known the way the awards went this evening, he could have taken it as tribute," she said in her acceptance speech.
Earlier bookmakers had touted short story writer Ali Smith as the likely winner, but in the end her book, "The Accidental," and Tash Aw's Malaysian-set saga "The Harmony Silk Factory" were not in the final three.
Michael Murpurgo, chairman of the judges, said Spurling beat off strong competition from poetry winner Christopher Logue's "Cold Calls," a modern reworking of Homer's "Iliad" and Kate Thompson's children's book, "The New Policeman."