The first Mustang generation was 1965-'66, with the Mustang introduced on April 17, 1964, as a '65 model. The Mustang didn't change much for 1966, with minor trim changes. During the first five months, there were two body styles-hardtop and convertible. In August 1964, the fastback was introduced as a third body style. These three body styles remained Mustang mainstays through 1973.