The Northern Irishman has been a regular in the Leeds side during his time in Yorkshire and made his 12th consecutive appearance for the Whites in Saturday's goalless draw at Reading.
McCartney told United's official website: "I'm here until the end of the month now.
"I wanted to come here and get back playing. We were a bit up and down at the start but we're on a roll now. We're more solid at the back."