McCartney will go straight into Simon Grayson's squad for Saturday's derby clash with Sheffield United at Elland Road.
Grayson said: "We're delighted to welcome George. He has good experience, he strengthens the squad and gives us increased competition for places."
McCartney sustained knee ligament damage in Portugal in the summer and has yet to appear this season for Sunderland, whom he made 25 appearances in the top flight for during the last campaign.
The 29-year-old signed for the Black Cats from West Ham in September 2008 in a £4.5million deal and will now be vying with summer signing Fede Bessone and teenager Aidy White for a starting place at left-back for Leeds.