What Was Lost Was Lost" was the winner in the first novel category of the Costa Book Awards. O'Flynn received a £5,000 prize. It was short-listed for the overall Costa Book of the Year Award. The Costa Book Awards' judging panel, chaired by Joanna Trollope, praised the novel for "blending humour and pathos in a cleverly constructed and absorbing mystery." They described the novel as inventive, compelling, and poignant. "What Was Lost" was long-listed for the 2007 Man Booker Prize and the Orange Prize for Fiction. It was short-listed for the Guardian First Book Award. It won the Jelf Group First Novel Award
In the Light of What We Know – but for our century.” Rebecca Mead of "The New Yorker" wrote that the novel was "talky and intellectual, while also unfolding a riveting drama: a deeply satisfying book," and that it was remarkable, “a 21st-century novel written with the ambition of scope of a 19th-century novel, and bearing the seriousness of purpose of a 20th-century one.” "In the Light of What We Know" earned its author the prestigious James Tait Black Memorial Prize, Britain's oldest literary prize. It was long-listed for the Orwell Prize 2015, the Guardian First Book award 2014, the Authors’ Club Best First Novel Award 2015,