The British scientist said he thought his wife was joking when she told him he had won the prize for his work on magnetic resonance imaging (MRI).
"I didn't expect anything like this at all," he said on Monday. "If someone just told you, you had won the Nobel prize, I think the reaction of 90% of the population would be 'Yeah, go on, pull the other one'."
Mansfield won the prize jointly with Paul Lauterbur of the US for their discovery, which enables doctors to see the internal organs and other tissues through the use of a strong magnetic field.
The scientist said his team of physicists working on MRI did not realise the impact their achievements would have.
"At those early stages one isn't thinking about the hundreds of millions of people who might benefit from it. You are thinking about getting the damn thing to work," the soft-spoken scientist said.
Mansfield had no intention of entering medicine. He was a rocketry buff who quit school at 15 and studied part time to get to university where he hoped to learn how to make missiles.
In the early 1970s he started using magnetic fields to study crystals. At one point he was recording images of plates of solid material separated by Vaseline. He says he realised: "That is exactly what biological material is."
University of Illinois professor
Paul Lauterbur was also honoured
He and his team began recording images of plants and bits of animal tissue. But when the time came to test their technology on a live subject, none of them was a biologist and they had no experience with laboratory animals.
Mansfield decided to volunteer himself. "Today I think we wouldn't be allowed to do that experiment," he said with a sly smile.
He was not scared at the beginning, but admits he later worried whether he might have done damage to his health.
He had nothing to fear. Three decades later, MRI is one of the safest and most widely used diagnostic techniques in the world. Sixty million people had MRI hospital scans in 2002.
Because the technology was so revolutionary, Mansfield's team built much of its own equipment without the backing of large medical technology companies. He owns many of the engineering patents and has done well in business.
But he prides himself on using his resources to further research in his field. He donated a new building to the university, where a brand-new plaque marks the Sir Peter Mansfield Magnetic Resonance Centre.
Peter Morris, a colleague of nearly 30 years who succeeded Mansfield as the centre's head, said he was little surprised to see his mentor win science's most prestigious honour.
"Given the impact it has had on medicine I guess it is a surprise there hasn't been a Nobel prize for it before," he said.