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.

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.

Although bearing a family resemblance to the XK120 and XK140, the XK150 was radically revised. A one-piece windscreen replaced the split screen, and the wing line no longer dropped so deeply at the doors. The widened bonnet opened down to the wings, and on the Roadster the bulkhead was moved back 4 inches (102 mm) to make the bonnet longer. The car was available at various times in Red, Pearl Grey, White, Indigo Blue, Claret, Cotswold Blue, Black, Mist Grey, Sherwood Green, Carmen Red, British Racing Green, Cornish Grey, and Imperial Maroon.

The XK140’s walnut dashboard was replaced by one trimmed in leather. On the early Drophead Coupés, the aluminium centre dash panel, which was discontinued after June 1958, had an X pattern engraving similar to the early 3.8 E-type. Thinner doors gave more interior space. On the front parking lights, which were located atop the wings (fenders), a little red light reminded the driver the lights were on.

The standard engine, the same as fitted to the XK140, was the 3.4 litre DOHC Jaguar straight-6 rated at 180 SAE bhp at 5750 rpm but most cars were fitted with the SE engine whose modified cylinder head and larger exhaust valves boosted the power to 210 SAE bhp at 5500 rpm. Twin 1.75-inch (44 mm) SU HD6 carburettors were fitted.

While the first XK150 were slower than their predecessors, the deficit was corrected in the spring of 1958 with a 3.4-litre “S” engine with three 2-inch (51 mm) SU HD8 carburettors and a straight-ported cylinder to give a claimed 250 SAE bhp.

For 1960, Jaguar bored the 3.4 litre engine to 3.8 litres, rating this option at 220 hp (164 kW; 223 PS) in standard tune or 265 hp (198 kW; 269 PS) in “S” form. A 3.8 litre 150S could top 135 mph (217 km/h) and go from 0–60 mph in around 7.0 seconds, thus restoring whatever verve the XK had been missing. Fuel economy was 18mpg.
Four-wheel Dunlop 12 in (305 mm) disc brakes appeared for the first time although it was theoretically possible to order a car with drums. Either wire wheels or disc wheels could be specified. Suspension and chassis were very similar to the XK140. Steering was by rack and pinion; power steering was not offered. Production ended in October 1960, and totaled 2265 Roadsters, 4445 Fixed Head Coupés and 2672 Drophead Coupés.