The land here was once part of a much larger Crown Commons grazing area called Slieau Lhoost - sometimes called 'the Cairns' - that ran along the ridge of Slieau Ree at 316 metres above sea level. After the Act of Revestment in 1765, when the British Crown bought the feudal rights from the Duke of Atholl, this upland passed into the hands of the UK's HM Commissioners of Woods and Forest. In 1933 the Isle of Man Government bought the 1,165-acre block for £2,000, specifically to fence the road and remove the two physical mountain gates that gave Keppel Gate its name, in time for the 1934 Isle of Man TT. From 1926 the grazing tenancy was held by mountain shepherd G. Rhodes Tate, who lived with his wife Gladys in a cottage at the adjacent 34th Milestone. Their home was once known as Tate's Cottage. It is now famous, slightly mispronounced, as Kate's Cottage.

The A18 Mountain Road was assembled in pieces during the 19th century from older horse paths, ancient rights of way, and incomplete carting tracks across peat bog and hillside. The Disafforesting Commission of 1860 opened up sales of Crown Common land in 1863, and the next few years saw new road sections built from the proceeds, partly subsidised by the Commissioners of Woods and Forest. The Ordnance Survey of 1867-8 shows a fresh stretch of road from Creg-ny-Baa across the steep slope of Slieau Ree to a shepherd's hut near what is now Kate's Cottage. The new alignment bypassed an older mountain track that survives today as a public footpath leading to Windy Corner. The familiar small and large metal milestone markers along the road date from the period of James Garrow, the island's Survey-General of Highways.

The Keppel Gate stretch was already part of motor racing on the island before motorcycles ever attempted it. The 1904 Gordon Bennett Trial used it as part of the 52.15-mile Highroads Course for cars. The RAC Tourist Trophy car races of 1905-1907 came through, and the 1908 Four Inch Course - named for a 4-inch cylinder-diameter limit - included it too. When the Auto-Cycle Club adopted the same circuit for motorcycles in 1911, the Keppel Gate section was already part of the lap. The course became the Snaefell Mountain Course, 37.73 miles long, used continuously for the TT since 1911 and the Manx Grand Prix since 1923. By 1921 the Auto-Cycle Union was so unhappy with the corner that it threatened to move the whole TT to Spa-Francorchamps in Belgium. The Manx Highway Board responded by tearing up the narrow stretch from Windy Corner to Keppel Gate and rebuilding it for the 1922 races. The race stayed.

Each safety improvement at Keppel Gate has been paid for. The widening of the section from the Thirty-Third Corner to near Keppel Gate, completed for the 1947 TT, followed the death of Peter M. Aitchison during the 1946 Senior Manx Grand Prix. The major road foundation rebuild during the winter of 1991-92 replaced tarmac laid in the early 1920s and closed the road for months. The 2006-07 winter saw a high-grip 'Shell-mac' resurfacing for road racing. In August 2009 a section of grass bank was removed from the southern side of the corner to provide a run-off area, following a practice crash by Australian TT winner Cameron Donald and a more serious incident at the same spot involving Travelling Marshal John McBride. In April 2015 the Highways Section took out another grass bank on the north-eastern side and reprofiled the road again. Each change was a small concession in a long argument between the road and the people who race on it.

For 50 weeks a year, Keppel Gate is quiet upland. The heath and heather moorland here belong to the North Atlantic sub-montane heath ecosystem, recognised as a European 'priority habitat' under Annex 1 of the EU Habitats Directive, and the Manx government has designated the area an Area of Special Scientific Interest under the Isle of Man Wild Life Act of 1990. Specialists call it a biodiversity hot-spot. Walkers on the public footpath from Windy Corner pass cairns that have weathered there for unrecorded centuries. The TT Marshal's stone shelter at Guthrie's Memorial up the road was demolished in the winter of 2012-13. Sheep still graze, watched over by descendants of the tenancy that began with Rhodes Tate in 1926. From here, the view runs west across Baldwin Valley, with Beinn-y-Phott to the north and on a clear day the dark line of the Mountain Course curving away into the distance.