The first running in 1923 was billed as a meeting for amateurs, a learning ground for riders who could not yet compete with the professionals on the Snaefell Mountain Course. Defining what an amateur actually was, however, turned out to be impossible. The rules were extensive and open to interpretation. By 1930 the event was simply called the Manx Grand Prix, and the philosophical question of amateur status was effectively abandoned. The format settled. The MGP became the proving ground for riders aiming at the Isle of Man TT in May or June: lap the Mountain Course in late August or early September, on smaller machines or older ones, learn the corners between the stone walls and the hedgerows, and earn your way to the senior event.

The course begins at the TT Grandstand in Douglas, runs north past Quarter Bridge, climbs through Glen Helen and over Sulby, crosses the bleak shoulder of Snaefell, and drops back through Creg-ny-Baa to Douglas: 60.70 kilometres of regular public road, closed for racing under the Road Races Act 2016. Newcomers, the most distinctive class of the meeting and the only one of its kind in international road racing, must wear coloured bibs over their leathers during practice, often orange or yellow, so that experienced riders can identify and give them space. On the first practice evening, Newcomers are escorted around the circuit on a speed-controlled lap by the Travelling Marshals, a roving fraternity of marshals on bikes who lap the course regularly to spot problems. After that, riders circulate at their own pace, working their way toward the qualifying speeds the regulations demand.

In 1989 Gloria Clark became the first woman to race in the Manx Grand Prix. Two years later she entered the Guinness Book of Records as the fastest lady on the TT Circuit. Twenty years on, in the 2009 Manx Grand Prix, Carolynn Sells did something nobody, male or female, had quite done in the modern era from her position: she won the Ultra-Lightweight race outright, becoming the first female winner on the Snaefell Mountain Course. Guinness logged that record too. These were not parade laps. Sells beat the rest of the field over multiple laps of an unforgiving circuit at racing speed.

The MGP is officially the amateur's race, but the names who came up through it include some of the most famous in motorcycling. Freddie Frith, Phil Read, and the great Geoff Duke all rode here on their way to multiple world championships. Joey Dunlop, with twenty-six TT victories the second-most successful TT rider of all time, came back to the MGP after his TT years on a classic Aermacchi and took a podium finish. Past Classic Junior winners include Michael Dunlop, Lee Johnston, Michael Rutter, Jamie Coward, and Dominic Herbertson. The winner of the Senior Manx Grand Prix receives the A.B. Crookall Trophy, but only once: convention obliges the senior winner to graduate to the TT proper. Forty-two separate awards are handed out across the meeting, funded entirely by entry fees and donations to the Manx Motor Cycle Club.

Since 2013, the Manx Government Department of Economic Development has expanded the event into what it calls the Festival of Motorcycling, adding a Classic TT category for historic machines and allowing professional riders to compete on vintage bikes. The format runs over nine days now, down from the traditional fortnight. There is no funfair on Douglas Promenade, the way there is for the TT. Instead there are club meetings, classic parades on closed roads, and the Manx three-day trial. The 100th anniversary of the event was celebrated in 2023. The MGP is what the TT was before it became a global brand: a road race on the same roads, with the same risks, but with more space to breathe between the helmets.