Mario Illien (born 2 August 1949) is a Swiss engineer specialising in motorsport engine design and comes from Chur in the Canton of Graubünden, Switzerland. Despite living in a country in which motor racing was banned in 1955 (when Illien was six years old), he developed an interest in the sport during the 1960s while following the career of Jo Bonnier, a Swedish expatriate living in Switzerland.