The clockwise course begins at the TT Grandstand on Glencrutchery Road in Douglas. From there it descends Bray Hill before turning right at Quarter Bridge onto the A1 Douglas to Peel Road, running through the villages of Union Mills, Glen Vine, Crosby, and Greeba. The route then turns right at Ballacraine onto the A3, threading through Kirk Michael, Ballaugh, and Sulby before intersecting the A18 Snaefell Mountain Road at Ramsey. The A18 carries the course back toward Douglas over the highest point of the circuit — Hailwood's Height, just past the Bungalow at approximately 422 metres above sea level — before descending through the residential outskirts of Douglas to the finish line. The start and finish positions on Glencrutchery Road are fixed; the finish line is the one element of the course that has never moved through any modification.
There are 219 corners on the course, approximately 60 of which carry names, many commemorating riders who competed or died there.
Motor racing on the Isle of Man began in 1904 with the Gordon Bennett Trial for automobiles, which required the use of public roads after the UK Motor Car Act 1903 imposed a 20 mph speed limit on mainland Britain. The Tynwald passed the Highways (Light Locomotives) Act 1904 to permit racing on island roads. Motorcycle events followed from 1905. The Mountain Course took its essential shape from 1911, using what was then known as the Four-Inch Course after a 1908 regulation limiting engine cylinder diameters to four inches. The official course distance was formalised at 37.73 miles in 1938 and stands at 37.730 miles today.
Between the wars, substantial road improvements shaped the Mountain Course. Changes in 1919 altered the Mountain section and established the current start and finish on Glencrutchery Road. Major modifications for the 1934 TT removed a sheep-gate on the East Snaefell Mountain section. Road widening programmes through the 1950s and 1960s addressed corners including Ballaugh Bridge, Barregarrow, and sections of Sulby Straight. A roundabout at Quarterbridge was added in 1963. In the 1970s road widening cut into the hillside at the Verandah section.
From 2003 to 2006 road repair work covered the A3 from Barregarrow to Cronk-y-Voddy. The Department of Transport undertook road widening at Windy Corner in 2004–05 and Brandish Corner in 2005–06. In 2007–08 a new roundabout scheme at Braddan Bridge incorporated the historic Jubilee Oak Tree, and a further road project linked Signpost Corner to Governor's Bridge via the Bemahague Road. Major junction works at Quarterbridge from 2008 involved demolishing the Quarterbridge Hotel.
The first named corner was Edges Corner in 1920. Notable named sections include Birkin's Bend at Kirk Michael, named after Archie Birkin who died there in 1927 when he swerved to avoid a fish van; Handley's Corner near the 11th Milestone, named after Wal Handley following his crash in the 1932 Senior TT; and Guthrie's Memorial on the A18 Mountain Road, where a stone monument was built in 1939 for Jimmie Guthrie.
Later additions honour: Hailwood's Rise and Hailwood's Height, named in 1981 for Mike Hailwood; Duke's at the 32nd Milestone for six-times world champion Geoff Duke; and Joey's at the 26th Milestone for Joey Dunlop, who won 26 TT races. In 2013 corners were named for active riders John McGuinness and Dave Molyneux. More recently, Caley's Corner at the entrance to Sulby Straight was named for local resident Ray Caley, who died in 2017.
The outright lap record is 16 minutes 36.115 seconds at an average of 136.358 mph (219.447 km/h), set by Peter Hickman on a BMW M1000RR Superstock bike during the 2023 Superstock TT Race Two. The Senior TT race lap record — 16 minutes 42.778 seconds at 135.452 mph — also belongs to Hickman, set during the 2018 Senior TT. The sidecar lap record of 18 minutes 42.350 seconds at 121.021 mph was set by Ryan Crowe and passenger Callum Crowe during the 2025 Sidecar TT Race 1.
In practice for the 2006 TT, New Zealander Bruce Anstey recorded an unofficial top speed of 206 mph on Sulby Straight, registered by onboard data logging.
The Mountain Course has been used for cycle racing since the 1930s, including the Manx International road race from 1936 and the 2017 British National Road Race Championships. The cycling absolute lap record was set in 2015 by Peter Kennaugh in 1 hour, 23 minutes and 48 seconds. Cars have also lapped the course: Mark Higgins completed a Subaru WRX STI lap in 17 minutes 35 seconds in 2016 at an average of 128.73 mph; the first 100 mph average-speed car lap was achieved by Tony Pond in 1990.
Between 1911 and 2025 there have been 270 rider fatalities during official practices or races on the Mountain Course, including deaths recorded during the Manx Grand Prix and Clubman TT events. The first fatality was Victor Surridge in 1911 near Glen Helen. From 1928 all practice sessions were held on closed roads following the death of Archie Birkin in 1927. The 1970 TT was the deadliest single year, with six competitor deaths including world championship contender Santiago Herrero. The 1972 death of Gilberto Parlotti in the 125 cc race led to the introduction of weather-contingent postponement rules the following year. Progressive safety upgrades from the 2000s onward have included mandatory course licences, newcomer speed control laps, air-fencing, and GPS tracking on course inspection vehicles.