During the early years of the twenties, while the Side Valves were making steady progress, the company started working on a new light car which was to transform its fortunes. Percy Riley, the engineering genius of the family, developed the power unit, the famous overhead valve, twin camshaft, four cylinder 1098cc 9hp engine, while brother, Stanley, designed the bodywork. Both engine and body were revolutionary and were to have great influence on contemporary manufacturers.

The first outing of the prototype was seen by an admiring public at the Shelsley Walsh Hill Climb in 1926 and over the next eleven years many thousands were sold on a world wide basis and in the car's many forms. The Riley Nine was an acclaimed market leader, eclipsing significantly anything that the company had done before. It set new standards and fashions and was soon 'discovered' by the sporting fraternity as a car that could be race tuned to astonishing levels. It achieved many successes in competition and in pioneering ventures around the world where its ruggedness and reliability became legendary.

Riley (Coventry) Ltd. went from strength to strength, developing engines of various sizes and layout while body styles ranged from the out and out sports, Brooklands Model, to the beautiful sports saloons of the mid thirties and luxurious 'Autovia' limousine. It was perhaps Riley's desire to be all things to all men that in the end over extended the company and in February 1938 the inevitable happened and, tragically, the Receiver was called in. The golden years when, between 1926 and 1938, some 127 different models were produced, were sadly over and Riley passed into the hands of Lord Nuffield.