Deeley was born at Sandwell General Hospital, West Bromwich, England and grew up in Sutton Coldfield and Great Barr. She attended Grove Vale Junior School followed by Dartmouth High School in Great Barr, where she played the clarinet in the Sandwell Youth Concert Band. She then joined the sixth form at Bishop Vesey’s Grammar School in Sutton Coldfield.
At age 14, Deeley entered a regional edition of a BBC competition for The Clothes Show, in which she reached the national finals. Spotted by an agent for Storm modeling agency at age 16, she was quickly signed as a model under her nickname of Cat to make it easier for clients to re-book her.