The Royal Automobile Club wanted a British answer to the Monaco Grand Prix, but the law was uncooperative: closing public roads for racing was illegal on the British mainland. The Isle of Man, however, made its own laws, and it had already lent its roads to motorcycle racing for the TT. The R.A.C. drew up a circuit through the streets of Douglas and organized two events for July 1933: the Mannin Beg, or Small Man, for unsupercharged voiturettes under 1500cc on the twelfth, and two days later the Mannin Moar for cars over 1500cc and supercharged voiturettes. The same streets that hosted Tynwald and the Manx parliament would, for forty-eight hours, host Bugattis and Maseratis.

The R.A.C. insisted on a rule that almost no one wanted. Every car had to carry a riding mechanic alongside the driver. The official purpose was communication: the mechanic could signal flag marshals when his driver wanted to overtake the car ahead, and the marshals would wave the slower car aside. Earl Howe, writing on behalf of the drivers to The Motor magazine, called the rule an embarrassment that would reduce a serious international event to second-rate status. The R.A.C. held its ground. This was almost certainly the last time the riding-mechanic requirement was used in Grand Prix racing anywhere in the world.

The first track measured 7.4 kilometres. Drivers ran up Greensills Corner to Church Road, hooked into a sharp left on Finch Road, fired past the House of Keys, climbed Prospect Hill, snaked through six rapid bends across Ballaquale Road, St Ninians Road, Dukes Road, and Falcon Terrace, then dropped to Governor's Bridge in a fast left-hander. From Onchan they came howling down Summer Hill Road between a stone wall on one side and Victorian terraces on the other, then ran along the promenade past Castle Mona back to the start. The 1934 course was shortened to 5.889 kilometres, and 1935 lengthened to 6.494. What stunned visitors was the speed. The Douglas circuit produced an average lap speed of about 105 kilometres per hour, compared to Monaco's 96. Brian Lewis, who would later become the 2nd Baron Essendon, won all three runnings.

The Mannin Moar ran in 1933, 1934, and 1935, and then it stopped. The reasons were practical: declining entries, the looming pressures of the late thirties, and a sport that was professionalizing past the small Manx street race. But for a brief moment Douglas had been a Grand Prix venue, with crowds lining the promenade and engines howling between the boarding houses. The street names on the route remain: Bucks Road, Prospect Hill, Summer Hill Road, Governor's Bridge. Drive them today in a normal car at normal speeds and they reveal themselves as ordinary urban streets. Then read the lap times from 1933, and imagine a supercharged voiturette taking the same corners flat-out, with a mechanic strapped in beside the driver, ready to wave the next slower car aside.