The exterior of the XK140 distinguished itself from the XK120 with more substantial front and rear bumpers with overriders, and flashing turn signals above the front bumper. The grille remained the same size but became a one-piece cast unit with fewer, broader, vertical bars, incorporating the Jaguar badge into the surround. A chrome trim strip ran along the centre of the bonnet and boot lid, and an emblem on the boot lid contained the words "Winner Le Mans 1951–3".
Interior improvements included moving the engine, firewall and dash forward to give 3 inches (76 mm) more legroom for taller drivers. The number of batteries varied by body style; Fixed Head Coupes had two 6-volt batteries, while Drop Heads and the Open Two Seater had a single 12-volt battery. The car was powered by the 3.4 litre Jaguar XK double overhead camshaft inline-6 engine, with the Special Equipment modifications raising the specified power to 190 bhp (142 kW) gross at 5500 rpm as standard. The optional C-Type cylinder head produced 210 bhp (157 kW) gross at 5750 rpm.
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. Wire wheels and dual exhausts were optional, with most XK140s imported into the United States having the optional wheels. Cars with standard disc wheels had spats over the rear wheel opening.
Three body styles were available: the Open Two Seater (OTS) or Roadster, the Drophead Coupé (DHC), and the Fixed Head Coupé (FHC). The OTS featured a light canvas top that folded out of sight behind the seats, with removable canvas and plastic side curtains. The DHC had a lined canvas top that lowered onto the body, a fixed windscreen, wind-up side windows, and a small rear seat. The FHC shared the DHC's interior trim and rear seat, but had a lengthened roof, a windscreen placed further forward, shorter front wings, and longer doors, all resulting in easier entry and more interior space.
Performance testing by Sports Cars Illustrated in August 1957 recorded a fastest two-way average speed of 121 mph (195 km/h), with a best one-way run of 124 mph (200 km/h). Acceleration from 0–60 mph (97 km/h) was recorded at 8.4 seconds and 9.1 seconds by Road & Track and Sports Cars Illustrated respectively. A standing 1/4 mile (~400 m) took 16.6 seconds (82 mph approx) and 16.9 seconds (86 mph).
Coachbuilt versions of the XK140 were produced by Zagato and Ghia. A Jaguar XK140 Zagato Coupé was built for Guido Modiano after his XK140 was damaged in an accident, and two more were subsequently built, although these were based on the XK150 chassis. Giovanni Michelotti designed the Jaguar XK140 Coupé by Ghia in 1955, with three cars reportedly built.