Actress Siân Thomas was born in Stratford-upon-Avon, Warwickshire, England, on 20 September 1953, and she spent part of her childhood living in Canada. She is the sister of the actress Sara Mair-Thomas. Her partner is the British poet Tony Harrison.
Siân Thomas is fluent in English, which is her primary language. It is likely that she also understands and speaks several other European languages, such as German, French, Spanish, Italian, and possibly some sign languages. However, due to limited information, her fluency in these languages cannot be definitively confirmed.