Northampton were quickly out of the blocks and Brennan Dickenson latched onto Alan Connell's pass only to be denied by Alan Marriott while Chris Hackett also tested the experienced keeper with a free-kick.
But Hackett was forced to clear Ryan Tafazolli's header off the line and Lee Stevenson was off target as Mansfield threatened. The visitors broke the deadlock in the 32nd minute when Mathias Kouo-Doumbe was undone by Matt Rhead's clever pass and Oliver Palmer beat the advancing Matt Duke.
Cobblers went close to an equaliser before the break with Ricky Ravenhill blazing over, Hackett having a goal-bound shot deflected over and Kouo-Doumbe heading wide at the near post.
Connell fired over after the restart and substitute Ian Morris forced Marriott to tip his fierce drive over from the edge of the box. Chris Wilder's men finally drew level in the 64th minute from the penalty spot after Hackett's cross was handled by Darryl Westlake, who was sent off, and Carter beat Marriott from the spot.
Despite being down to ten men, Mansfield were still a threat, Anthony Howell shot into the side-netting from a tight angle and James Jennings fired over. But Hackett missed a great chance in the closing stages when he volleyed Connell's cross over.