The 29-year-old Northern Ireland international is back at Elland Road after making 17 appearances in a successful loan spell before Christmas.
McCartney, who has made 203 appearances during two spells at the Stadium of Light, is likely to be named in the squad for Saturday's Championship clash with Scunthorpe but is unavailable for the FA third-round replay with Arsenal on Wednesday.
Simon Grayson told the club's official website: "I'm delighted we've been able to bring George back until the end of the season. He did well over the course of his last loan spell and I know what he can bring to the table."
Ken Bates added: "The additional revenue earned from the Arsenal game has helped make this loan possible as we continue to invest on and off the field."
Meanwhile, Slovakia centre-back Lubo Michalik has left Leeds by mutual consent.