The production car changed little from the prototype, although the full-width rear bumper was dropped in favour of two part-bumpers curving round each corner, with overriders. Mechanics were basically stock Herald components: The engine was a 4-cylinder of 1,147 cc, mildly tuned for the Spitfire with twin SU carburettors. Also from the Herald came the rack and pinion steering and coil-and-wishbone front suspension up front, and at the rear a single transverse-leaf swing axle arrangement. This ended up being the most controversial part of the car: it was known to "tuck in" and cause violent over steer if pushed too hard, even in the staid Herald. In the sportier Spitfire it led to severe criticism.
The body was bolted to a much-modified Herald chassis, the outer rails and the rear outriggers having been removed; little of the original Herald chassis design was left, and the Spitfire used structural outer sills to stiffen its body tub. The Spitfire was an inexpensive small sports car and as such had very basic trim, including rubber mats and a large plastic steering wheel. These early cars were referred to both as "Triumph Spitfire Mk I" and "Spitfire 4", not to be confused with the later Spitfire Mark IV. From 1964 an overdrive option was added to the four speed gearbox to give more relaxed cruising. Wire wheels and a hard top were also made available.
In March 1965 the Spitfire Mk II was released and was very similar to the Mk I but featured a more highly tuned engine, through a revised camshaft design, a water cooled intake manifold and tubular exhaust manifold, increasing the power to 67 bhp (50 kW) at 6,000 rpm. This improved the top speed to 92 mph (148 km/h). The exterior trim was modified with a new grille and badges. The interior trim was improved with redesigned seats and by covering most of the exposed surfaces with rubber cloth. The original moulded rubber floor coverings were replaced with moulded carpets. Top speed was claimed to be 96 mph (154 km/h) and its 0-60 mph time of 15.5 seconds was considered "lively."
The Mk III, introduced in March 1967, was the first major facelift to the Spitfire. The front bumper was raised in response to new crash regulations, and although much of the bonnet pressing was carried over, the front end looked quite different. The rear lost the overriders from the bumper but gained reversing lights as standard; the interior was improved again with a wood veneer instrument surround. A folding hood replaced the earlier "build it yourself" arrangement. For most of the Mk III range, the instrument cluster was still centre-mounted so as to reduce parts bin counts for RH and LH drive versions.
Starting in 1969, however, US-bound models were produced with a "federal" dashboard design which moved the gauges in front of the driver, a layout that would be adopted for all markets with the Mk IV. The 1,147 cc engine was replaced with a bored out 1,296 cc unit, as fitted on the new Triumph Herald 13/60 and Triumph 1300 saloons. In twin-carburettor form, the engine put out a claimed 75 bhp (56 kW) and made the MK III a comparatively quick car by the standards of the day. Popular options continued to include wire wheels, a hard top and a Laycock de Normanville overdrive, and far more relaxed and economical cruising at high speeds. The Mk III was the fastest Spitfire yet, achieving 60 mph (97 km/h) in 12.5 seconds and actually continued production into 1971, well after the Mk IV was introduced.
The MK IV brought the most comprehensive changes to the Spitfire. It featured a completely re-designed cut off rear end, giving a strong family resemblance to the Triumph Stag and Triumph 2000 models, both of which were also Michelotti-designed. The front end was also cleaned up, with a new bonnet pressing losing the weld lines on top of the wings from the older models, and the doors were given recessed handles. The interior was much improved: a proper full-width dashboard was provided, putting the instruments ahead of the driver rather than over the centre console. The engine continued at 1,296 cc, but was modified with larger big-end bearings to rationalize production with the TR6 2.5 litre engines, which somewhat decreased its "revvy" nature; there was some detuning, to meet new emissions laws, which resulted in the new car being a little tamer than the MK III. The gearbox gained synchromesh on its bottom gear. By far the most significant change, however, was to the rear suspension, which was de-cambered and redesigned to eliminate the unfortunate tendencies of the original swing-axle design.
In 1973 in the US and 1975 for the rest of the world, the 1500 engine was used to make the Spitfire Mk IV 1500; though in this final incarnation the engine was rather rougher and more prone to failure than the earlier units, torque was greatly increased which made it much more drivable in traffic. The reason for the engine problems was due to continued use of three main bearings for the crank shaft. The US market models were considerably less powerful than the British market cars because they had to meet more stringent US emissions requirements. While the rest of the world saw 1500's with the compression ratio reduced to 8.0:1, the American market model was fitted with a single Zenith Stromberg carburettor and a compression ratio reduced to 7.5:1 to allow it to run on lower octane unleaded fuel and after adding a catalytic converter and exhaust gas recirculating system, the engine only delivered 53 bhp (40 kW) with a 0-60 time of 14.3 seconds.
Detail improvements continued to be made throughout the life of the MK IV, and included reclining seats with head restraints, wood-veneer dash, hazard flashers and electric washers. Options such as the hard top, tonneau cover, map light and overdrive continued to be popular, though wire wheels ceased to be available. The 1980 model was the last and the heaviest of the entire run weighing in at 1,875 lb (850.5 kg). The last Spitfire, an Inca Yellow UK-market model with hardtop and overdrive, rolled off the assembly line at Canley in August 1980, shortly before the factory closed.