He was walking near Slieve Binnian on Monday when he fell at about 16:30 BST. Other walkers spotted him and raised the alarm.
A team of 18 people from the Mourne Mountain Rescue service went to his assistance.
Martin McMullan, coordinator with the rescue team said the man had been in a place known as the water slabs.
"He was coming near the top of the area which is very slippery and quite challenging," he said.
"It seems that he lost his footing at the top and then slid down part of the water slabs and started to tumble before he came to rest at the bottom.
"He had taken quite a fall and sustained multiple injuries primarily to the upper part of his body. We treated him for head, neck and chest injuries as a result of that fall."
Mr McMullan said the man slid and tumbled between 30 to 50 metres.
After he was treated at the scene for his injuries, he was transported by helicopter to Belfast's Royal Victoria Hospital where his condition was described as "serious".
Mr McMullan said that even the most skilled individuals can have accidents in the mountains and stressed the importance of having a good back-up emergency plan before setting out.
A man has been airlifted to hospital in Belfast after falling up to 50 metres in the Mourne mountains.