The Series 1 was introduced, initially for export only, in March 1961. The domestic market launch came four months later in July 1961. The cars at this time used the triple SU carburetted 3.8-litre six-cylinder Jaguar XK6 engine from the XK150S. Earlier built cars utilised external bonnet latches which required a tool to open and had a flat floor design. These cars are rare and more valuable. After that, the floors were dished to provide more leg room and the twin bonnet latches moved to inside the car. The 3.8-litre engine was increased to 4.2 litres (4,235 cc) in October 1964.
The 4.2-litre engine produced the same power as the 3.8-litre (265 bhp;198 kW) and same top speed (150 mph;241 km/h), but increased torque approximately 10% from 240 to 283 lb?ft (325 to 384 N?m). Acceleration remained pretty much the same and 0 to 60 mph (0 to 97 km/h) times were around 6.4 seconds for both engines, but maximum power was now reached at 5,400 rpm instead of 5,500 rpm on the 3.8-litre. That all meant better throttle response for drivers that did not want to shift down gears. The 4.2-litre's block was completely redesigned, made longer to accommodate 5 mm (0.20 in) larger bores, and the crankshaft modified to use newer bearings. Other engine upgrades included a new alternator/generator and an electric cooling fan for the radiator.