The Mini 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. Leonard Lord, the somewhat autocratic 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 feet; and the passenger accommodation should occupy 6 feet of the 10-foot length; and the engine, for reasons of cost, should be an existing unit.

Alec Issigonis had been recruited back to BMC in 1955 with a brief from Lord to design a range of technically advanced family cars in the same innovative spirit as his earlier Morris Minor. With Lord's dictum to produce a bubble car competitor and his revised design requirements being laid down in October 1956, work on the smallest of these became his priority. The team that designed the Mini was remarkably small; in addition to Issigonis, John Sheppard, Jack Daniels, Chris Kingham, two engineering students, and four draughtsmen worked on the project. Together, by July 1957, they had designed and built the original XC9003 prototype. Leonard Lord approved the car for production on 19 July and XC9003 became project ADO15.