The book, which is a biography of Alan Greenspan, one of the titans of financial history over the last fifty years, met the award's criterion of providing the "most compelling and enjoyable insight into modern business issues".
Mallaby was awarded the prize at a ceremony at the National Gallery here earlier this week.
"'The Man Who Knew' is an impressive work of scholarship. It is a masterpiece of political economy, and above all it is a great and enjoyable read," Lionel Barber, editor of the Financial Times, said.
He shows the subtlety and complexity of Alan Greenspan's legacy.
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Appointed by Ronald Reagan to Chairman of the Federal Reserve, a post he held for eighteen years, Greenspan presided over an unprecedented period of stability and low inflation, was revered by economists, adored by investors and consulted by leaders from Beijing to Frankfurt.
The book is replete with "beautifully rendered high-octane political infighting, hard hitting dialogue and stories".
"It is well researched, enormously gripping and the story of the making of modern finance."