The left-back, 29, has fallen out of favour at the Stadium of Light and will provide Leeds with cover in the absence of the injured Federico Bessone.
Sunderland boss Steve Bruce has confirmed that he will not stand in the way of the deal despite losing defenders Anton Ferdinand and Kieron Richardson to injury in the midweek Carling Cup defeat by West Ham United.
McCartney is likely to make his debut in Saturday's Yorkshire derby against Sheffield United at Elland Road.