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.

Several athletes have come under scrutiny after stolen medical records showed they used banned substances for medical reasons under TUE rules.
Briton Froome, 31, said the system is "open to abuse" and must be addressed "urgently" by cycling's governing body, the UCI, and the World Anti-Doping Agency (Wada).
"I have never had a 'win at all costs' approach in this regard," the Team Sky rider said in a statement.
"I am not looking to push the boundaries of the rules.
"I believe that this is something that athletes need to take responsibility for themselves, until more stringent protocols can be put in place."
Froome and Sir Bradley Wiggins, a former team-mate at Team Sky, were among the athletes whose medical records were made public by hacking group Fancy Bears.
There is no suggestion any of the athletes named have broken anti-doping rules.
Froome, whose TUEs for prednisolone in May 2013 and April 2014 were revealed, said: "There are athletes who not only abide by the rules that are in place, but also those of fair play.
"I take my position in the sport very seriously and I know that I have to not only abide by the rules but also go above and beyond that to set a good example both morally and ethically."
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Wiggins, Britain's most decorated Olympian and the first British winner of the Tour de France in 2012, has been criticised for the timing of his TUEs, shortly before major races in 2011, 2012 and 2013.
He said he took the anti-inflammatory drug triamcinolone for allergies and respiratory issues, telling the BBC on Sunday that he was not trying to gain an "unfair advantage" from being allowed to use an otherwise banned steroid.
Team Sky boss Sir Dave Brailsford on Monday said the team did not "do not cross the line" over performance-enhancing drugs and that he had complete trust in their doctors.
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They have also beaten up people wearing red T-shirts, the colour of the opposition Movement for Democratic Change party (MDC), a BBC reporter at the scene says.
Protesters are calling for electoral reform ahead of polls in 2018.
President Robert Mugabe, 92, says he intends to stand again.
Leaders from 18 opposition political parties called for Zimbabweans to march through Harare as part of a so-called "mega demonstration".
The High Court ruled on Friday morning that the planned opposition march could go ahead, a day after police warned that unauthorised demonstrations would not be tolerated.
Public protests have proliferated in recent months, focusing on the dire state of the country's economy.
Most of these have come under the banner of the #ThisFlag movement, inspired by charismatic pastor Evan Mawarire, who has urged non-violent protest over perceived corruption and economic mismanagement
The country's economic crisis has worsened recently, leading to a chronic cash shortage and delays paying civil servants.
Home Affairs Minister Ignatius Chombo blamed a demonstration that ended violently on Wednesday on foreign elements.
"We cannot sit idly while our country is being torn apart by unruly foreign-sponsored agents," Zimbabwe's state-run Herald newspaper quotes him as saying.
A man is in a critical condition in hospital after falling up to 50 metres in the Mourne Mountains.
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Tour de France winner Chris Froome says athletes must "take responsibility" and not "push the boundaries of the rules" following criticism of the therapeutic use exemption system.
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Police in Zimbabwe have fired tear gas and water cannon at opposition supporters who had gathered for a protest march in the capital, Harare.