Hewlett was born in London and grew up between the west coast of Ireland and Whitstable in Kent. Her father was the comedy sitcom actor Donald Hewlett, known for his roles in It Ain't Half Hot Mum and You Rang, M'Lord?. Her mother is the Actress Therese McMurray. Hewlett attended Wellesley House School, Downe House School and The King's School, Canterbury on art scholarships. Whilst at The King's School, Canterbury, she was spotted in a production by the theatrical agent Lorraine Hamilton and subsequently studied acting at Webber Douglas Academy of Dramatic Art in London, graduating in 2003.