The XK120 was launched as an open two-seater or (USA) roadster at the 1948 London Motor Show as a testbed and show car for the new Jaguar XK engine. The display car was the first prototype, chassis number 660001. It looked almost identical to the production cars only that the direct external pillars of its windshield would be curved in the production version. The sports car caused a sensation, which persuaded Jaguar founder and president, William Lyons, to put it into production.

Starting in 1948, the first 242 cars used open 2-seater bodies with wooden frames and aluminum panels.

Production shifted to the 51st heaviest in the early 1950s. The "120" in the name referred to the aluminum car's top speed of 193 km / h (faster with the windshield). removed), which made it the fastest production car in the world at the time of its launch. In 1949, the first production car, chassis number 670003, was delivered to Clark Gable.

The first production XK120, chassis number 670003 originally owned by Clark Gable, at the 2012 Pebble Beach Concours d'Elegance. The XK120 was the fastest production car in the world at the time of its debut.

The XK120 was finally available in three versions or body styles, first as a 2-seater opener described in the US market as a roadster (OTS) and as a fixed head coupe (FHC) from 1951 and finally as a dropup coupe ( DHC) from 1953, all two seats and are available with Left (LHD) or Right Hand Drive (RHD).

A smaller engine version with a 2-liter 2-cylinder engine, designated the XK100, intended for the UK market, was canceled before production.

On May 30, 1949, on the empty Ostend-Jabbeke highway in Belgium, a prototype XK120, programmed by officers of the Royal Automobile Club of Belgium, achieved an average of runs in opposite directions of 132.6 mph with the windshield replaced by just a small aero screen. and a cataloged alternative top gear ratio, [note 1] and 135 mph with a passenger side tonneau cover in place. [7] In 1950 and 1951, at the Linas-Montlhéry Autodrome, an inclined oval track in France, the open XK120 averaged 100 mph for 24 hours and more than 130 mph for an hour. In 1952, a fixed-head coupe achieved countless world records for speed and distance when it averaged 160 km / h for a week.

The XK120s were also highly successful in racing and rallying.