Envisioned as a luxury sports car, the Stag was designed to compete directly with the Mercedes-Benz SL class models. All Stags were four-seater convertible coupés, but for structural rigidity – and to meet proposed American rollover standards of the time – the Stag required a B-pillar "roll bar" hoop connected to the windscreen frame by a T-bar. A body colour removable hard top with defrost wires on the rear window, full headliner and lever operated quarter windows was a popular factory option.

The car started as a styling experiment by Michelotti cut and shaped from a 1963–64 pre-production 2000 saloon, also styled by Giovanni Michelotti. His agreement was, if Harry Webster, Director of Engineering at Triumph, liked the design, Triumph could use the prototype as the basis of a new model. Webster loved the design and took the prototype back to England. The result, a two-door drop head, had little in common with the styling of the 2000, but retained the suspension and drive line. Triumph liked the Michelotti design so much that they incorporated the styling lines of the Stag into the new T2000/T2500 Mark II saloon and estate model lines of the 1970s

Webster intended the Stag, large saloons and estate cars to use a new Triumph-designed overhead cam (OHC) 2.5-litre fuel injected (PI) V8. In 1968 the new 2.5 PI V8 was enlarged to 2,997 cc to increase the power available.

The car was launched nearly two years late in June 1970, to a warm welcome at the various international auto shows. In the UK the Stag was an immediate success for Triumph with a 12-month waiting list rapidly being established and cars changing hands at well above list price. but when it was released into the US, its main target market, it rapidly acquired a reputation for mechanical unreliability, usually in the form of overheating.  A number of owners have replaced the troublesome engine with units from other cars, such as the Rover V8, or the Triumph 2.5-litre engine around which the Stag was originally designed. The number of such conversions undertaken is not known, but as at July 2017, 91% of Stags known to DVLA had a 3-litre engine, according to www.howmanyleft.com. It is not clear how many of these are original Stag engines and how many are Ford 3-litre Essex units. The once-popular Rover V8 conversion powers fewer than 5% of surviving Stags and it is thought by the relevant owners club that the Ford engine figures are much lower than this.

In the end this is another potentially great car from British Leyland spoilt by lack of funds to develop it properly and the appalling build quality of British Leyland cars.