Siobhan Dowd was born to Irish parents and brought up in London. She spent much of her youth visiting the family cottage in Aglish, County Waterford and later the family home in Wicklow Town.

She attended a Catholic grammar school in south London and then gained a degree in Classics at Lady Margaret Hall, Oxford University. After a short stint in publishing, she joined the writer's organisation PEN, initially as a researcher for its Writers in Prison Committee.