United have not recorded a victory in the competition for five years and were forced to a replay after Andy Robinson's penalty merely cancelled out Scott McGleish's opening goal for the Cobblers.
Giles Coke was dismissed for a second bookable offence in the 27th minute, but Gary McAllister's side were unable to find a way through.
Veteran striker McGleish, on loan to Northampton from Wycombe, scored after only nine minutes with a far post tap-in after Ryan Gilligan and former Elland Road striker Leon Constantine had created the chance.
But former Mansfield midfielder Coke did the Cobblers no favours by earning a second yellow card and dismissal in the 27th minute when he brought down Fabian Delph. Town were also affected by an early injury to their captain Chris Doig who was replaced by Alex Dyer in the 12th minute.
Leeds, without leading scorer Jermaine Beckford, had started tentatively but they made the extra man count by dominating possession. When Mark Hughes tripped Jonny Howson just inside the penalty box, Andy Robinson drove the ball high into the net from the penalty spot after 37 minutes to level the score.