The first "Jaguar" model was offered in 1935, and after WW2 Lyons changed the company name to Jaguar to avoid the unfortunate connotations with the Nazi "SS". Armstrong Siddeley allowed William Lyons to use the Jaguar name from their successful aircraft engine range, such was the camaraderie of the car industrie at the time.

During the War vehicle production was switched to aircraft manufacture and repair, but engineering development did continue. Some secretive military projects were undertaken but most importantly for the future of the company, Lyons and his engineering team worked on a new engine which was to power his vision of a mass-produced sporting saloon car. The XK engine was completed in 1948 and launched in a (supposedly) one-off concept sports car to help draw attention to it. This succeeded far better than was envisaged and both became an overnight sensation, globally. The XK engine went on to power all Jaguars until the introduction of the series 3 E Type introduced the Jaguar V12 engine in 1971, while the XJ6 continued in production until 1992 with the 4.2 liter version of the XK engine. The sports car, XK120, went into full production too and led to a string of attention-grabbing (and profitable) sports cars which led to international sporting success (most notably at Le Mans) and helped put the name of Jaguar Cars and Coventry on the world map. But Lyons main focus was on the saloon car which he continued to develop until his last and proudest achievement, the XJ6 of 1968.

During his time as managing director of Jaguar, Lyons could be best described as 'autocratic' and kept a tight rein on the company. It is said that board meetings were rare until the 1960s. He was responsible for the styling of every new model introduced (although the C-type, D-type, E-type and XJ-S were designed by Malcolm Sayer). This was remarkable, as Sir William was not a trained draughtsman, and designed primarily using full scale 3-D mockups, which were continually adjusted by craftsmen working under his instructions. Undoubtedly one of his other great skills was to pick the highly accomplished team that was to remain loyal to him for so long. William Heynes, Claude Bailey, Walter Hassan, Harry Mundy, Harry Weslake and many others all contributed to create a strong engineering team