Scott Taylor ended his sixth-month goal drought to put the Dons ahead shortly after the break, but Arnold crashed in a fantastic 30-yard drive in the 90th minute to earn the lowly Stags a share of the spoils.
Mansfield started brightly and Martin Gritton should have done better when he headed Michael Boulding's pin-point cross straight at Dons goalkeeper Adolfo Baines from six yards in the fourth minute.
The Dons took control and Jon-Paul McGovern's curling 30-yard free-kick was clawed out of the bottom corner by full-stretch Mansfield keeper Jason White in the 24th minute, after Gareth Jelleyman had been yellow carded for scything down Gary Smith.
Yet the Stags threatened on the counter-attack and Boulding's angled low 16-yard drive was well held by keeper Baines after a powerful burst down the left flank in the 28th minute.
Milton Keynes continued to dominate possession but they struggled to make any real inroads into the well-drilled Mansfield defence, as Leon Knight's difficult 12-yard header over the bar from Drissa Diallo's 37th minute cross was the only other attempt at goal they mustered in the first half.
The Dons burst out of the blocks after the break with some quicker incisive football and it paid dividends immediately as Taylor put them in front just four minutes after the re-start.
McGovern's dangerous low cross was only half-cleared to Taylor 10 yards out, whose strike deflected off Alex-John Baptiste and past keeper White for only his second goal of the season.
The Stags battled back and Gritton's 17-yard angled drive was well saved one-handed by Dons keeper Baines, before Jonathan D'Laryea lashed just wide from 30 yards.
Mansfield piled on the pressure late on and, after Arnold's long range 88th minute drive had flown inches wide, he went one better at the death when he turned sharply 30 yards out and unleashed a devastating right-foot strike into the bottom corner of the net.