Born in Swansea, Davies aspired to work as a full time comic artist in his adult life, until a career advisor suggested that he study English literature; he consequently refocused his aspirations to play- and screen-writing. After he graduated from Oxford University, Davies joined the BBC's children’s department on a part-time basis in 1985 and worked in varying positions, including producing and writing two series, Dark Season and Century Falls, until leaving in 1994 to become a freelance writer.
Davies moved into writing adult television dramas in 1994. His scripts generally explored concepts of religion and sexuality among various backdrops: Springhill was a soap drama about a Catholic family in contemporary Liverpool; The Grand explored society’s opinion of subjects such as prostitution, abortion, and homosexuality during the interwar period; and Queer as Folk, his first prolific series, was set in the Manchester gay scene and recreated his own experiences living as a gay man in Manchester. His later series include Bob & Rose, which portrayed a gay man who fell in love with a woman, the The Second Coming, which focused on the second coming and deicide of Jesus Christ, Mine All Mine, which portrayed a Swansea family’s discovery that they owned their home city, and Casanova, an adaptation of the Venetian lover’s complete memoirs.
His most notable achievement was reviving and running the science fiction series Doctor Who after a sixteen year hiatus, casting Christopher Eccleston and later David Tennant in the title role of the Doctor. Davies' tenure as executive producer of the show oversaw a surge in popularity that led to the production of two spin-offs, Torchwood and The Sarah Jane Adventures, and the revival of the Saturday primetime slot as a profitable venture for production companies. Davies was awarded an OBE in 2008 for services to drama, which coincided with his announcement that he would step down from as the show’s executive producer with his final script, The End of Time (2009-10). Davies is currently living in Los Angeles, California and continues to oversee production of Torchwood and The Sarah Jane Adventures.