Imogen was born to Charles Thomas, an education welfare officer, and his wife Janette in Gorseinon, near Swansea, South Wales. Her parents separated when she was two years old, and, after her mother remarried when she was six, she was brought up by her mother and step-father in Llanelli, South Wales. Thomas has a sister, Alana. Thomas developed alopecia when she was six, losing chunks of her hair.
Thomas has been working as a model since she was 13, and at the age of 17 moved to London to become a professional model. Working in bars to pay for her living costs, she was rejected by many modelling agencies due to her height.
She reluctantly returned to her studies, and gained a degree in Health studies and Psychology at the University of Worcester. During this period she entered beauty contests, and was crowned Miss Wales in 2003, beating future Big Brother 9 winner Rachel Rice, who came third. She went on to represent Wales at Miss World in China. This prompted her to return to London to try glamour modelling, and also working as a bar hostess at London's Sanderson Hotel.