McCartney re-joins the club he spent two years with back under Alan Curbishley’s reign at the Boleyn Ground.
McCartney will provide competition for current left-back Hertia Ilunga.
Speaking to the official site George admitted that it was good to return to familiar grounds.
“It can be tough moving to a new club but I’ve been here before. I know most of the staff who are still here and the team is quite similar to when I left it - there are still quite a lot of the lads here.
“It’s all about me getting my bearings again and going out to prove to the manager, players, fans and myself that I’m still the player I was three or four years ago.”