The Mitsubishi Galant GTO was launched in 1970 as the two-door hardtop variant of Mitsubishi's then-new Galant sedan, and was known as the Colt Galant GTO. Designed by Hiroaki Kamisago, who had previously been sent by Mitsubishi to study at the Art Center College of Design in Pasadena, California, it incorporated many stylistic cues from contemporary American muscle cars like the Mustang, Firebird and Cougar, including a long hood, raised cut-off ducktail rear, and rounded quad-headlamps and tail-lamps. It was also the first Japanese passenger car to have full side windows and a pillarless design.

There were three variants available at first, all powered by the Saturn engine: the M1 (1600 cc SOHC, 4-speed), M2 (1600 cc SOHC, 5-speed) and the top-spec MR (1600 cc twin-carb, DOHC 5-speed), a 125 hp (92 kW) version only available in Japan.

In 1972 Mitsubishi upgraded the powerplants with their new Astron units. The range now consisted of the LS (2000 cc single-carb, automatic transmission), GS (2000 cc twin-carb, 5-speed manual) and GS-R (2000 cc twin-carb, 5-speed manual). They were also given a mild facelift to distinguish them, comprising a one-piece slats-type grille and three-piece tail lights. Additionally, the GS-R had wider 185-section tires, flared guards and a black-painted rear panel between the lights.

There was a second styling tweak in 1974 when the car gained a honeycomb-style front grille. Also, some of the very last cars gained the Astron 80 engine with Mitsubishi's Silent Shaft system, before the entire range was discontinued the following year in favour of the Lancer-based Celeste and the Galant Lambda/Sapporo.

After production ceased, the name lay dormant for fifteen years, but it retained sufficient cachet that Mitsubishi resurrected it for their flagship Mitsubishi GTO sports GT in 1990. However, in order to avoid offending automotive connoisseurs, who might have objected to the evocative nameplate from the highly regarded Ferrari 250 GTO and Pontiac GTO being used on a Japanese vehicle, it was renamed to Mitsubishi 3000GT overseas.