The success interrupts Magenta's three-year monopoly on the title and means Mont Dore will be New Caledonia's representatives in the 2012 O-League.
The club last appeared in the competition in the inaugural 2007 season where they put in a sixth-place showing.
Coach Percyl Avock and his men booked their return to the Pacific's biggest club stage with goals from Cesar Lolohea and Michel Hmae against Magenta but it was the losing side who got on the scoresheet first.
Magenta were on a roll after two wins in their last two games - an O-League victory over Tefana and a shock triumph in the French Cup against USL Dunkerque - and carried that form into the must-win league clash with Mont Dore by going ahead in the 38th minute when a Pierre Wajoka corner resulted in an own goal.
But Mont Dore were determined to finally get one over their local rivals and were also in red hot form, having won their last five games in a row.
They started the second half by putting more pressure on the Magenta defence and missed a great opportunity to score when Lolohea could not beat the goalkeeper in a one-on-one.
But they were not to be denied for much longer and earned the equaliser their spell of pressure deserved in the 67th minute as Lolohea made up for his earlier miss by slotting home a penalty.
Mont Dore continued to have the better of the game and found the winning goal just eight minutes later when a mistake by Magenta goalkeeper Michel Hne, who otherwise had an outstanding game between the sticks, let Hmae in to score and book his side's ticket to next year's O-League.