Aptly for a stage performer, Sian was born in Stratford-on-Avon, sister of fellow actress Sara Mair-Thomas, though she spent part of her childhood in Canada. After graduating in acting from the Central School of Speech and Drama, she spent a year in rep before working for two years at the Glasgow Citizens' Theatre, which she describes as one of the happiest times of her life. Shortly after she made her debut as "Katarina" in the Royal Shakespeare Company's production of "The Taming of the Shrew" and has since worked with several leading stage groups, notably the National Theatre, where she met her partner, the director and writer Tony Harrison. She also co-starred with Madonna in the American performer's West End debut in "Up For Grabs" and it is no secret that several critics thought she rather upstaged the then-Mrs. Ritchie. Sian, however, recalls her time with Madonna as very enjoyable and would be happy to work with her, again. On television, she has, in company with most actors in Britain, done the rounds of mystery thrillers, such as Midsomer Murders (1997), Taggart (1983) and The Bill (1984).