The XK140 was introduced in late 1954 and sold as a 1955 model. Exterior changes that distinguished it from the XK120 included more substantial front and rear bumpers with overriders, and flashing turn signals (operated by a switch on the dash) above the front bumper.

The grille remained the same size but became a one-piece cast unit with fewer, and broader, vertical bars. The Jaguar badge was incorporated into the grille surround. A chrome trim strip ran along the centre of the bonnet (hood) and boot (trunk) lid. An emblem on the boot lid contained the words “Winner Le Mans 1951–3”.

The interior was made more comfortable for taller drivers by moving the engine, firewall and dash forward to give 3 inches (76 mm) more legroom. The single battery was relocated from behind the seats to inside the wing (fender) on the inlet side.
The XK140 was powered by the Jaguar XK engine with the Special Equipment modifications from the XK120, which raised the specified power by 10 bhp to 190 bhp (142 kW) gross at 5500 rpm, as standard. The C-Type cylinder head, carried over from the XK120 catalogue, and producing 210 bhp (157 kW) gross at 5750 rpm, was optional equipment.

When fitted with the C-type head, 2-inch sand-cast H8 carburettors, heavier torsion bars and twin exhaust pipes, the car was designated XK140 SE in the UK and XK140 MC in North America.
In 1956 the XK140 became the first Jaguar sports car to be offered with automatic transmission. As with the XK120, wire wheels and dual exhausts were options, and most XK140s imported into the United States had wire wheels. Cars with the standard disc wheels had spats (fender skirts) over the rear wheel opening.

The XK120 was launched in roadster form at the 1948 London Motor Show as a testbed and show car for the new Jaguar XK engine. The display model was the first prototype, chassis number 670001. It looked almost identical to the production cars except that the straight outer pillars of its windscreen would be curved on the production version. The roadster caused a sensation, which persuaded Jaguar founder and design boss William Lyons to put it into production. In 1949 the first customer car, chassis number 670003, was delivered to Clark Gable.

The “120” in its name referred to its 120 mph (193 km/h) top speed (faster with the windscreen removed), which made the XK120 the world’s fastest standard production car at the time of its launch. It was available in two open versions, first as the roadster (designated OTS, for open two-seater, in America), then also as a drophead coupé (DHC) from 1953; and also as a closed, or “fixed-head” coupé (FHC) from 1951. The DHC was a more deluxe open model, with wind-up windows, and wood-veneer dashboard and interior door caps, as on the FHC. A smaller-engined version intended for the UK market was cancelled prior to production.

In 1950 and 1951, at a banked oval track in France, XK120 roadsters averaged over 100 mph for 24 hours and over 130 mph for an hour, and in 1952 a fixed-head coupé took numerous world records for speed and distance when it averaged 100 mph for a week. Roadsters were also successful in racing and rallying.