Designated by Leonard Lord as project ADO15 (Austin Drawing Office project number 15) the Mini actually came about because of a fuel shortage caused by the 1956 Suez Crisis. Petrol was once again rationed in the UK, sales of large cars slumped, and the market for German bubble cars boomed. Lord, the somewhat dictatorial head of BMC, reportedly detested these cars so much that he vowed to rid the streets of them and design a 'proper miniature car'. He laid down some basic design requirements: the car should be contained within a box that measured 10 †'— 4 †'— 4 ft (3 †'— 1.2 †'— 1.2 m); and the passenger accommodation should occupy 6 ft (1.8 m) of the 10 ft (3 m) length; and the engine, for reasons of cost, should be an existing BMC unit. Alec Issigonis, who had been working for Alvis, had been enticed back to BMC in 1955 and, with his skills in designing small cars, was a natural for the task. The team that designed the Mini was remarkably small: as well as Issigonis, there was Jack Daniels (who had worked with him on the Morris Minor), Chris Kingham (who had been with him at Alvis), two engineering students and four draughtsmen. Together, by October 1957, they had designed and built the original prototype.

The ADO15 used a conventional BMC A-Series, four-cylinder engine but it departed from tradition by having it mounting it transversely. the four-speed transmission was lubricated by the engine-oil-in the sump. This layout allowed front-wheel drive and despite meeting the stringent dimensional constraints of Leonard Lord, passenger room was impressive. The radiator was mounted at the left side of the car so that the engine-mounted fan could be retained, but with reversed pitch so that it blew air into the natural low pressure area under the front wing.