For the London Motor Show in October 1966 the Mark X was renamed the Jaguar 420G (not to be confused with the smaller Jaguar 420). The 420G was distinct from the Mark X only with the addition of a vertical central bar splitting the grille in two, side indicator repeaters on the front wings, and a chrome strip along the wing and door panels (two tone paint schemes were also available with the chrome strip omitted). Interior changes included perforations in the central sections of the leather seats, padded dashboard sections for safety, moving the clock to a central position, and the introduction of air conditioning as an option.
A "limousine" version was available, on the standard wheelbase, with a dividing glass screen partition and front bench seat replacing the separate seats of standard cars. The wheelbase was extended by 21", with the mechanical underpinnings of the car being subtly reconfigured, for the 1968 Daimler DS420. This car was built until 1992 and used by many countries in official capacities, and frequently by funeral homes; either with a saloon body for carrying mourners or a hearse body.
Despite running for the same length of time as the Mark X (five years) the 420G sold in less than a third of the numbers: this lack of popularity and the increasing production of the XJ6 resulted in the 420G being run out of production in 1970.