Illien was born in Switzerland - a country where motor racing was banned when Mario was six, as a result of the 1955 Le Mans disaster. Nonetheless Illien developed a passion for racing and it was the exploits of Swedish driver Jo Bonnier, who had made his home in Switzerland, which drew him to the sport. Trained as a technical draughtsman he went to work for Bonnier in 1971, helping to run an old McLaren. After Bonnier was killed at Le Mans Illien was 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 would go on to be used in Formula 2 in the mid-1970s but by then Illien had enrolled to study mechanical engineering at the famous Biel University Engineering School. He graduated in 1977 and joined the Mowag company in Kreuzlingen, designing diesel engines for tanks and armored vehicles.Mario's passion for the sport did not go away and at 30 he gave up his job and 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 identified a demand for a good new Indycar V8 turbo engine. They sought funding from Roger Penske and he found them support from General Motors offshoot Chevrolet. Illien, Morgan, Penske and GM took a 25% share each in a new company - Ilmor Engineering, with Morgan looking after the manufacturing and commercial side of the business and Illien designing.The Ilmor Chevrolet V8 ran for the first time in August 1985 and won two pole positions in 1986. Between 1987 and 1993 the engines won 86 races, six Indianapolis 500s and five Indycar titles. In 1989 Illien and Morgan decided to build a 3.5-liter V10 for Formula 1. This was used by Leyton House in 1991 and by March and Tyrrell in 1992. The 1993 season saw Ilmor design a Sauber V10 (paid for by Mercedes) and at the end of the year Mercedes-Benz bought out Chevrolet's share of Ilmor and the Sauber V10 became the Mercedes V10. In America Ilmor built a special unit for Mercedes at Indianapolis and Al Unser Jr won the 1994 Indy 500 with ease.For 1995 Mercedes dropped Sauber and went into partnership with McLaren. The result was not an immediate success but in America Ilmor-designed Mercedes engines scored six wins that year. The first Mercedes V10 victory in F1 came finally at the end of 1997 and the McLaren-Mercedes combination then dominated the 1998 and 1999 seasons as well, with Mika Hakkinen becoming Champion on both occasions. The company was less successful in the United States although Greg Moore scored three wins in that period before his death at the end of the 1999 season.