The man 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 helped the man before he was airlifted to hospital.
The man had been in a place known as the water slabs, said Martin McMullan, a rescue team coordinator.
He described the area as very slippery and quite challenging.
"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," said Mr McMullan.
"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."
The Mourne Mountain Rescue Service was also involved at the weekend in the search for the two missing crewmen of an Irish Coast Guard helicopter that crashed off Blacksod in County Mayo.
Both men have been missing since Rescue 116 crashed on 14 March.
A man is in a critical condition in hospital after falling up to 50 metres in the Mourne Mountains.