A qualified solicitor, she previously worked at Ladbrokes as head of the firm's in-house gambling division before joining law firm Berwin Leighton Paisner as a partner in 2000.
Stewart-Jones joined DLA Piper in 2011 and remained as a consultant to the company before departing the partnership in May this year.
Alan Jackson, non-executive chairman of Playtech, said: "Since her appointment, Hilary has made a significant contribution to the company's board through her extensive knowledge of the gambling industry.
“On behalf of the board, I am delighted that Hilary has agreed to become my deputy.”