George joined Boro from Doncaster Rovers in July 2012 for an undisclosed fee, signing a three-year contract.
Boro beat off competition from two other Championship clubs to secure the highly-rated Doncaster captain, who was named Player of the Year and Player's Player of the Year at The Keepmoat Stadium in 2011-12.
Barnstaple-born George started his career at Exeter City and helped the West Country side regain their Football League status with a Conference play-off triumph over Cambridge United at Wembley in 2008.
George, who can play at either left-back or centre-back, made his debut in the 2-1 Capital One Cup win at Bury in August 2012.
The 2013/14 campaign was a memorable one for the popular defender, who scored three goals and was named Ramsdens Player of the Year, Middlesbrough Official Supporters Club Player of the Year, Community Player of the Year and Players’ Player of the Year.