After leaving school in 1942, he was first apprenticed to Rolls-Royce. He joined Rover, run by his uncles Maurice and Spencer Wilks, in 1945 and worked initially on the gas-turbine powered JET1 and T3 experimental prototypes. In 1959 he became chief engineer of new vehicle projects and is best known for his leadership of the teams that developed the advanced Rover P6 series, introduced as the 2000 in 1963, and the hugely successful Range Rover (of which a “CSK” special edition later celebrated his involvement) launched in June, 1970. As well, he was responsible for the Rover-based Marauder sports car in 1950 and many Rover experimental and prototype vehicles.
As Rover was first taken over by Standard-Triumph and then absorbed into British Leyland he also led teams responsible for the Triumph TR6, Triumph Stag and Triumph TR7 models as well as the innovative design of the 16-valve cylinder head used on the Triumph Dolomite Sprint. Several other Leyland Group models were developed under his leadership but during a chaotic time for the British motor industry, King was frequently frustrated by the design compromises imposed by lack of adequate funding, and the poor quality of vehicles produced by an uncooperative workforce in the diverse and mainly outdated plants owned by the company.
While chairman of BL Technology from 1979 he was responsible for developing a series of light, aerodynamic and technically advanced ECV (Energy Conservation Vehicle) experimental models, features of which were incorporated into later Leyland Group products such as the Rover K-series engine, or adopted by other manufacturers.
Spencer King retired from the company in 1985.
In 2004, he criticised SUV owners who drive their vehicles in urban areas, saying that vehicles like the Range Rover he created were "never intended as a status symbol but later incarnations of my design seem to be intended for that purpose."