The club’s co-owner Jamie Reuben has inspired the move and Thornton will be added immediately.
The club had previously stated that CEO Darren Eales was stepping down from his post for health reasons.
Newcastle statement read: "Born in South Shields, Roger is Head of Property at Motcomb Estates Limited, an asset management company that focuses on a portfolio of global office, retail, hotel and leisure assets for the family of Newcastle United co-owner, Jamie Reuben.
"Roger retains a keen interest in his native North East and has played a key role in recent developments in Newcastle city centre, including the transformative Pilgrims Quarter project, as well as supporting property matters for Newcastle Racecourse.
"He also has strong family links to the Magpies. His grandfather, Ben Sullivan, was an administrator who moved from Lincoln City to relaunch the Newcastle United Development Association in the mid-1960s.
"Ben played a key role in supporting ground improvements at St. James' Park over many years, including securing funding to build the existing East Stand structure which opened in 1973."