Sales began in November 1970, when it was the flagship hardtop variant of Mitsubishi Heavy Industries's then-new Colt Galant sedan.

The Colt Galant GTO exterior was penned by Hiroaki Kamisago, who had previously been sent by Mitsubishi to study at the Art Center College of Design, then located in Los Angeles, California.

The design incorporates a number of stylistic cues from contemporary American muscle cars such as the Mustang, Firebird and Cougar, including a long hood, raised cut-off ducktail rear, and rounded quad-headlamps and tail-lamps.

These cars received the A55C chassis code, while the twin-cam MR continued to use the smaller but more powerful 4G32 engine until January 1973, when stricter emissions standards made it obsolete.

[3] The lineup was also given a mild facelift to signal the changes, comprising a one-piece slats-type grille with a central dividing molding and three-piece tail lights.

This was followed by a second styling tweak in February 1975 when the car gained a honeycomb-style front grille, enough to be labelled the "New Galant GTO" in promotional material.