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, a result which leaves Simon Grayson's men eighth in the Championship table.
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."
Speaking about Leeds' targets for the season, he added: "It's too early too say what will happen in terms of promotion, there's two good teams at the top (QPR and Cardiff) and you would be happy with a play-off place. But we're still before Christmas and it's all about trying to get as many points as you can and see where that takes you."