Goals in quick succession by George Milner and Danny South meant the home side advance to the final, where they will face South Normanton Athletic.
Benito Lowe’s header in the 27th minute had put the visiting side ahead, but it wasn’t enough for Dave Birch’s side despite a spirited performance.
Both teams had chances to go in front early on - Belper’s George Milner tried his luck from a free-kick, testing Dylan Forth in the New Mills net, and just moments later Benito Lowe failed to connect with Will Ozono’s low ball up the other end.
Milner failed to divert Derry Robson’s cross on the 11th minute, before Lowe saw his effort trickle just wide after some neat footwork on the edge of the penalty area.
Just before the half-hour mark, New Mills took the lead through their dangerman, Lowe.
Jake Pollard’s floated free-kick was headed in by the striker to put the visitors ahead.
The visitors came close to doubling their lead when Tommy Lent latched onto Michael Norton’s header, before firing a half-volley into the ground and into the hands of Ryan Musselwhite.
Belper were handed an opportunity back into the game when referee, Reece Davies, gave the hosts a penalty after Nathan Curtis’s direct run ended with him being pulled down inside the area.
Milner stepped up and sent Forth the wrong way from 12-yards-out to equalise for the hosts.
The quick turnaround was complete two minutes later when Danny South headed Derry Robson’s cross into the net - the Belper captain was left unmarked to turn the game on it’s head.
After earning the penalty, Curtis came close to adding to The Nailers’ lead when his looping effort forced Forth into a brilliant save after linking up with South well.
After the break, it was Curtis again who came close for the hosts.
Again, it was South who was provider, as his lofted cross provided the winger with a free header, but the former-Bradford City academy player couldn’t direct his effort towards goal.
The victory now means Belper Town will face South Normanton Athletic in the final of the Senior Challenge Cup, at Pride Park Stadium on Wednesday 25th March.