Jessica Kelly Siobhán Reilly was born in Chessington, London, marking the beginning of her journey. While details about her early childhood are relatively scarce, it's known she developed an interest in acting at a young age, a passion that would eventually lead her to a successful career.

Reilly's career began with a significant break when she wrote to the producers of 'Prime Suspect,' requesting an audition. She landed a role in 'Prime Suspect 4: Inner Circles,' starring alongside Helen Mirren. This initial exposure provided a crucial launching pad for her future endeavors and introduced her to the world of professional acting.