The car started as a styling experiment literally cut and shaped from a 1963-4 Triumph 2000 pre-production saloon, which had also been styled by Michelotti, and loaned to him by Harry Webster, Director of Triumph Engineering from the early to late 1960's. Their agreement was under the premise that if Harry liked the design, Triumph could use the prototype as the basis of a new Triumph model. Harry, who was a long time friend of Giovanni whom he called "Micho", absolutely loved the design and spirited the prototype back to England. The end result, a two door drop head (convertible) had little in common with the styling of its progenitor 2000, but retained the suspension and drive line. Triumph liked the Michelotti design so much that they propagated the styling lines of the Stag into the new T2000/T2500 saloon and estate model lines of the 1970's.
It has been alleged that internal politics meant that Triumph intended, but were unable, to use the proven but old technology General Motors designed all aluminium Rover V8. As we know today, the no-fit story is probably a myth. Rover, also owned by British Leyland, simply could not supply the numbers of aluminium V8 engines to match the anticipated production of the Stag. It is also possible that General Motors would not give permission for Triumph to use the V8. We know that "brand loyalty" between Triumph and Rover was high between these former rivals. Besides, Harry Webster had already started development and testing of a new unique, all Triumph designed overhead cam (OHC) 2.5 litre fuel injected (PI) V8 to be used in the Stag, large saloons and estate cars. The vision was to allow Triumph to compete in the highly desired V8 marketplace. Under the direction of Harry's replacement, Spen King in 1968, the new Triumph OHC 2.5 PI V8 was enlarged to 2997 cc (3.0 litre) to increase torque and the troublesome fuel injection dropped in favour of dual Zenith-Stromberg 175 CDSE carburetors to meet emission standards of the target market - USA.
The Triumph Slant-4 engine shared the same basic design as the Triumph V8, consisting of a single overhead cam cast iron block with aluminium heads. However the cylinder heads of the two engines do not share the same footprint on their respective engine blocks. This same engine manufactured by StanPart was initially used in the Saab 99. Using a gear driven water pump, the Slant 4 could be easily installed in a front wheel drive car. This same water pump design was used in the Stag V8.
As in the Triumph 2000 model line, monocoque construction was employed, as was fully independent suspension—MacPherson struts in front, semi-trailing arms at the rear. Braking was by front disc and rear drum brakes, while steering was power-assisted rack and pinion.
British Leyland never provided sufficient budget to correct the few design issues of the Triumph 3.0 litre OHC V8. Many owners adopted a popular conversion of the car fitting a Rover V8, Ford Capri 2.8 V6, Buick 231 V6, or Triumph 6-cylinder engine but now such conversions fetch lower prices than a genuine Stag V8-engined car.
Perhaps thanks to such a reputation for trouble, only 25,877 cars were produced between 1970 and 1977. Of this number, 6,780 were export models, of which only 2,871 went to the United States. Several variants were produced noted only in changes of the production numbering sequences, these have become unofficially designated as "Early" 1970, the Mk I (1971–1972/3), Mk II (1973) and "Late" (1974-1977).
Most cars were fitted with a Borg-Warner 3-speed automatic transmission. The other choice was a derivative of the ancient TR-2 gearbox which had been modified and improved over the years for use in the TR4/A/IRS/TR5/250/6. First gear ratio was raised and needle roller bearings were used in place of the bronze bushings on the layshaft. Early models could be ordered with an A-type Laycock overdrive unit and later ones frequently came with a J-type Laycock unit. The overdrive option is highly desirable as the engine RPM is excessive without it.
Other than the choice of transmissions there were very few factory installed options. Some early cars came with just the soft-top and some with just the hard-top but most ended up with both. Electric windows, power steering and power assisted brakes were standard. Options included air conditioning, chrome wire wheels, luggage rack, Koni shock absorbers, floor mats and Lucas Square Eight fog lamps, and a range of aftermarket products, most of which were dealer installed as optional accessories. Rather unusually for a 4-seat touring car, the accessory list included a sump protector plate. This was probably included as a slightly 'gimmicky' tribute to Triumph's rallying successes.