Motor racing on the Isle of Man dates to 1904, when the island's Tynwald legislature passed the Highways (Light Locomotives) Act permitting the use of public roads for motorsport — an exception unavailable on the British mainland under the Motor Car Act 1903. The first car events used a 52-mile Highroads Course for the Gordon Bennett Trial. The TT races for motorcycles began in 1907 on the shorter St. John's Short Course after the mountainous A18 Snaefell Mountain Road proved too steep for the motorcycles of the era.
The present Mountain Course evolved from the Four Inch Course, named for the 1908 Tourist Trophy's engine-size regulations, and was first used for motorcycle racing in 1911. A significant realignment in 1919 redirected the course through Cronk-ny-Mona onto the A18 Mountain Road and established the Glencrutchery Road start and finish. The course length was fixed at 37.73 miles in 1938 and has remained essentially unchanged.
The lap begins in Douglas, descending urban streets before turning at Quarter Bridge onto the A1 Peel Road, then threading the villages of Union Mills, Glen Vine, Crosby, and Greeba. At Ballacraine the route switches onto the A3, climbing through Kirk Michael, Ballaugh, and Sulby before reaching Ramsey. From Ramsey, the A18 carries riders up to the open mountain plateau, cresting at Hailwood's Height near the 31st Milestone at 1,385 feet (422 m) above sea level. The long descent back into Douglas completes the lap. The course has approximately 219 corners, around 60 of which carry names.
Several corners are named in memory of riders or in honour of individuals connected with the event. Guthrie's Memorial on the A18 commemorates Scottish champion Jimmie Guthrie with a stone monument built in 1939. Hailwood's Rise and Hailwood's Height honour Mike Hailwood. Joey's at the 26th Milestone recognises Joey Dunlop's 26 race wins. Duke's at the 32nd Milestone is named for 1950s world champion Geoff Duke. Later additions honoured active competitors John McGuinness and Dave Molyneux in 2013, an unusual step for a circuit that traditionally named corners after fallen riders.
The outright motorcycle lap record stands at 16 minutes 36.115 seconds, an average of 136.358 mph (219.447 km/h), set by Peter Hickman on a BMW M1000RR Superstock machine during the 2023 Superstock TT Race Two. The Senior TT race lap record is 16 minutes 42.778 seconds at 135.452 mph (217.989 km/h), also held by Hickman from the 2018 Senior TT. The Sidecar TT lap record is 18 minutes 42.350 seconds at 121.021 mph (194.764 km/h), set by Ryan Crowe and passenger Callum Crowe in 2025.
The car lap record belongs to Mark Higgins, who lapped in 17 minutes 35 seconds at an average of 128.73 mph (207.17 km/h) in a 2016 Subaru WRX STI. In 2006 Bruce Anstey recorded an unofficial top speed of 206 mph (332 km/h) at the end of Sulby Straight on a 1000 cc Suzuki, registered by on-board data logging.
Continuous road improvement has marked the circuit's history. Major programmes in 1922 widened key sections including Sulby Bridge and the approach to Governor's Bridge. The 1934 and 1935 TTs saw removal of sheep gates and hump-backed bridges on the Mountain section. From 2003 to 2006 the A3 road from Barregarrow to Cronk-y-Voddy was extensively resurfaced. A major road-safety scheme in 2008 rebuilt the Quarterbridge junction. Safety improvements have progressively added air fencing, speed-reducing features at Mather's Cross, road widening at exposed sections, and modern run-off.
Between 1911 and 2025, 270 rider fatalities occurred during official practices or races on the Mountain Course, including deaths at the Manx Grand Prix and Clubman TT series. The first was Victor Surridge in 1911 near Glen Helen. Safety measures introduced over the decades include compulsory crash helmets from 1914, closed-road practice from 1928 onward, the seeding of competitors from 1959, mandatory helicopter weather clearance from 1973, and a Newcomers Speed Control Lap from 2004. Despite these improvements, the open-road nature of the circuit means that hazard elimination is inherently limited, and the TT remains a benchmark of danger within international motorsport.
The Mountain Course has also hosted cycle racing, including the Manx International road race from 1936. The cycling absolute lap record was set by Peter Kennaugh in 2015 at 1 hour 23 minutes 48 seconds, breaking Chris Boardman's 1993 mark by six seconds.