The judges were unanimous in choosing Roy's book, which was the bookmakers' 9-4 second favourite. Overcome with emotion, she said: "Like one of the children in my book I had learnt to control my hopes and hadn't made an acceptance speech."
Ms Roy went on to indicate that she may never write another book following her Booker win. She said: "The Booker Prize is about my past not my future. I will only write another book if I have another book to write. I don't believe in professions."
The author spent her early years in her grandmother's pickle factory in Kerala, southern India. After a degree in architecture, she had a variety of jobs including selling cakes on the beaches of Goa before turning to acting and writing screenplays.
Her husband is a film-maker in New Delhi, her father managed a tea estate and her mother is an advocate of women's rights and runs a chain of schools.
The book, at present the subject of an obscenity case in India, is a lyrical, tragi-comic novel set in and around a pickle factory in Kerala, has sold 400,000 copies worldwide. It unfolds against the lush south Indian landscape where twins come to terms with their mother's doomed cross cultural love match. It was described in The Independent as "a remarkably assured debut, both moving and compelling".
But it has not escaped controversy, sparking an obscenity trial in her native state over a love scene between a Christian woman and a low-caste Hindu man. Until this is settled, translations into India's principal languages have been put on ice.
Despite her win, this year's prize has been labelled by some critics the dullest and most uninspiring ever.
Over at Random House which fielded one front runner and an outside chance, the champagne was put back in the cupboard for the sixth year running. The losers, including the two flushed but decidedly unexhilarated authors, supped instead on beer and wine.