It was the home side who threatened in the opening period, and Dimitrios Konstantopoulos had to be alert in the 11th minute to make a good save from Nakhi Wells, and the Greek stopper was out quickly seven minutes later to smother the striker again.
The physical Championship clash saw a number of players go in with forceful tackles, and James Vaughan was shown a yellow card for his volatile reaction to what he perceived to be a foul on him.
Vaughan had to be replaced shortly after the incident with an injury as players from both sides continued to fly into tackles in a feisty encounter.
Six minutes later, Leadbitter sent a powerful free kick hurtling into the back of the net to put Middlesbrough 1-0 ahead, and the goal seemed to give the visitors confidence.
Kike was denied once more by Smithies shortly after the restart, and the miss proved to be costly as two of Powell's substitutes combined for Huddersfield's equaliser.
Huddersfield then had the chance to snatch all three points, but Wells's volley was too weak and a grateful Konstantopoulos gathered the loose ball.
Leadbitter made no mistake from the spot to give Boro a dramatic victory.