The course begins on Glencrutchery Road in the island's capital, Douglas, and immediately navigates urban streets before turning right at Quarter Bridge onto the A1 Douglas to Peel road. Competitors pass through Union Mills, Glen Vine, Crosby, and Greeba before turning at Ballacraine onto the A3, which leads through more open countryside and the villages of Kirk Michael, Ballaugh, and Sulby. After the town of Ramsey the course joins the A18 Snaefell Mountain Road for its highest section, crossing the exposed moorland plateau near the Bungalow tram crossing before the long descent back through the residential outskirts of Douglas to the finish.
The Mountain section — from Ramsey up the A18 to the summit plateau and back down — is the defining feature of the course. Competitors reach speeds in excess of 180 mph on sections like Sulby Straight, while the Mountain itself demands concentration across prolonged high-speed sweeps with no margin for error. The course passes private gardens, stone walls, telegraph poles, and kerbed pavements throughout the town sections.
The island's use of public roads for racing began in 1904 with the Gordon Bennett Trial for automobiles. A speed restriction of 20 mph under the UK Motor Car Act 1903 drove event organizers to the Isle of Man's autonomous legal framework, which permitted closed-road racing through its own Tynwald legislation. The original motorcycle races used a shorter St. John's Short Course in 1907; the four-inch course that evolved into the current Mountain Course route was formally adopted for motorcycle racing from 1911.
The present course length of 37.73 miles was confirmed in 1938 following gradual refinements to the road junctions and boundaries. The start-finish line shifted to its current position on Glencrutchery Road after 1919 course revisions extended the circuit from 37.50 miles to its near-final length.
Course changes since the early twentieth century have been incremental and focused primarily on safety: road widening at specific corners, removal of cottage buildings and obstructions, re-profiling of junctions, and the introduction of air fencing and safety barriers at hazard points. Major road widening programs occurred through the 1920s and 1930s, post-war improvements addressed the most dangerous sections, and from the 2000s onwards the Isle of Man Department of Infrastructure has continued systematic widening and re-surfacing projects.
Approximately 60 of the course's 219 corners carry names, many commemorating riders or incidents at those points. The practice of naming corners began with Edges Corner in 1920. Birkin's Bend in Kirk Michael commemorates Archie Birkin, killed there in 1927 after swerving to avoid a fish van — an incident that also prompted the closure of roads for all subsequent practice sessions from 1928 onwards. Guthrie's Memorial on the mountain road marks the site of a 1939 stone tribute to Jimmie Guthrie. Hailwood's Rise and Hailwood's Height, named in 1981, honor former world champion Mike Hailwood. Duke's at the 32nd Milestone commemorates 1950s champion Geoff Duke; Joey's at the 26th Milestone honors Joey Dunlop, who accumulated 26 TT victories. In 2013 the Isle of Man Government named corners after active competitors John McGuinness and Dave Molyneux.
The outright lap record for the TT Mountain Course is 16 minutes 36.115 seconds at an average of 136.358 mph, set by Peter Hickman on a BMW M1000RR during the 2023 Superstock TT Race Two. The Senior TT race lap record — 16 minutes 42.778 seconds at 135.452 mph average — is also held by Hickman, set during the 2018 Senior TT. The top speed record, unofficial, is 206 mph recorded in 2006 by Bruce Anstey at the end of Sulby Straight using on-board datalogging.
The sidecar lap record of 18 minutes 42.350 seconds was set by Ryan Crowe and passenger Callum Crowe during the 2025 Sidecar TT Race 1.
Between 1911 and 2025, 270 competitor fatalities have occurred on the Mountain Course during official practice or racing at the TT and Manx Grand Prix events. The first was Victor Surridge near Glen Helen during 1911 TT practice. The event's safety history has driven many of the incremental course changes over more than a century: compulsory crash helmets followed the 1913 death of Frank Bateman; closed-road practice resulted from the 1927 fatality at Rhencullen; helicopter availability became a race condition from 1973 following the 1972 death of Gilberto Parlotti. The year 1970 was the deadliest in the event's history, with six fatalities including world championship contender Santiago Herrero.
The Mountain Course has also hosted cycling events. The Manx International road race, held from 1936, is run on the same course. Peter Kennaugh broke the cycling lap record in 2015 with a time of 1 hour 23 minutes 48 seconds, surpassing a longstanding mark set by Chris Boardman in 1993. Car laps have also been recorded: Mark Higgins set successive records in 2011, 2014, and 2016 in Subaru WRX STI vehicles, ultimately achieving a 17-minute 35-second lap at 128.73 mph average.