The girl, a member of a Letterkenny Scouting group, was on the first day of an 80km fund-raising walk which was due to last three days.
However the scout dislocated her knee on rough ground in the Lough Sallagh area of the Derryveagh Mountains after 6pm.
A three man Donegal Mountain Rescue Team was rushed to the scene and reached the group and administered first aid to the girl.
The Sligo S61 Coastguard Helicopter was also scrambled to the scene along with a four man DMRT party and a SARDA Dog unit which were put on standby ready to be dropped into the area by helicopter.
Other DMRT members were also on route.
Despite fading light and deteriorating cloud conditions Rescue 118 managed to airlift the teenager by winch and flew her to Sligo Hospital for treatment.
The remainder of the scout group were walked off the mountain in darkness by DMRT and transported back to their anxious parents who were waiting in Glenveagh at the Visitors Centre Car Park around 11.30pm.
As well as Rescue 118 and Malin Head Coast Guard, DMRT wish to thank the Glenveagh Park Staff and Milford Garda for their support during the rescue.