At a height of , Great Dun Fell is the second-highest mountain in England's Pennines, lying south along the watershed from Cross Fell, its higher neighbour. Together with its smaller twin, Little Dun Fell, which reaches , it forms a stepping-stone for the Pennine Way on its long climb up from Dufton. It lies within the historic county boundaries of Westmorland, the ceremonial county of Cumbria, and the modern unitary authority area of Westmorland and Furness.
At the summit there is a radar station which is operated by National Air Traffic Services and is a key part of the Air Traffic Control system for Northern England and Southern Scotland. A radome containing Primary Surveillance radar (PSR) and Secondary Surveillance Radar (SSR) antennas, various towers and fencing crown the summit. Alfred Wainwright abhorred the old radio station (removed in the 1980s) in his book Pennine Way Companion.
The construction of the radar station led to the repaving of a tarred road to the summit, which became Britain's highest road. This road is marked as private from just above the village of Knock, and is not open to public motor vehicles. However, it is a bridleway until shortly before the radar station, so it is open to walkers, cyclists and horseriders.
In 2025 work commenced on the replacement of the radome.
The University of Manchester formerly had a permanent meteorological observatory at the Great Dun Fell site. It has hosted a number of field experiments doing research into clouds and their interactions with pollution. As the summit is in cloud for two thirds of the year it is an ideal location for this type of research. The university still has the option to use the site for short-term measurement periods.
There are the remains of hushing gulleys on the slopes of the mountain, created during lead mining of the Industrial Revolution. Most notable is the clearly visible hushing on the eastern flank of the fell.
On the west flank of the fell, immediately below the radar station, was Silverband Mine where galena was mined by the London Lead Company. Production changed to barytes in the early 20th century and in 1939, following the takeover of the mine by the Laporte Chemical Company, production was significantly increased. An aerial ropeway, which is now listed as a scheduled monument, was constructed to transport the ore down the fell for processing at Milburn Grange. The mine closed in 1966 but reopened in the 1980s and was worked sporadically. The mine is now fully closed although certain buildings remain following a significant land remediation project.
Under the Köppen climate classification, Great Dun Fell has a subpolar oceanic climate (Cfc), closely bordering both a subarctic climate (Dfc) under the isotherm and an alpine tundra climate (ET) due to a July mean of exactly . The Met Office station publishes only temperature, wind and frost averages. The summers are cool due to elevation. Considering its elevation and a latitude of over 54 degrees, winters are relatively mild due to oceanic influences.
The highest temperature recorded since 1963 was on 10 August 2003 and the lowest was on 8 February 1969.