It happened on the Glen River Valley side of Slieve Comedagh shortly after 14:00 BST on Sunday.
Mourne Mountain Rescue team and an ambulance service paramedic helped the man and his partner who was stranded on a ledge above.
The man was taken to a hospital in Belfast and treated for injuries to his legs, pelvis and ankles.
His injuries are not thought to be life-threatening.
A man has been flown to hospital by a rescue helicopter after falling 25-30 metres from cliffs in the Mournes.