The Stags’ 3-2 win at Field Mill was the first time they had beaten their neighbours in the league since September 1963.
The hosts raced into a 3-0 lead inside 19 second-half minutes.
The first half had seen County’s Mark Stallard miss a glorious brace of opportunities and his side was made to pay after the restart.
Andy White slid in at the far-post to convert Wayne Corden’s cross and send the Stags fans wild.
They were soon celebrating even more when sub and academy product Craig Mitchell scored a beauty.
The 17-year-old’s first senior goal was one to remember as he lobbed County’s Steve Mildenhall from 18 yards out.
Then on 71 minutes Corden made it 3-0 with a long-range curling effort.
With ten minutes to go County made a valiant effort at a comeback.
Stallard made amends for his earlier misses by notching a late brace but the Stags held on for a long-awaited success over their neighbours.
Stags: Pilkington, Mendes, Lawrence, Doane, Eaton, Corden, A White, Curtis, Gadsby, Day, Williamson.