Mario Illien was born in Switzerland. He trained as a technical draughtsman and went to work for Bonnier in 1971, helping to run an old McLaren. He was then hired by former racer Fred Stalder to modify a Chrysler-Simca four-cylinder engine for use in a Le Mans sports-prototype run by Stalder's Racing Organisation Course operation. This was used in Formula 2 in the mid-1970s at which time Illien had enrolled to study mechanical engineering at the famous Biel University Engineering School. He graduated in 1977 and joined the Mowag Company, designing diesel engines for tanks and armoured vehicles.
Mario had a passion for Motorsport and at 30 he went to work in the engine design department at Cosworth Engineering at Northampton in England. He spent the next five years designing and developing the company's engines, notably the DFY Formula 1 engine.
At the end of 1983 Illien and fellow Cosworth employee Paul Morgan founded Ilmor engineering along with Roger Penske. Mario took the position of Technical Director, leading the design of a succession of successful Indycar and F1 engines amongst other projects.