Au mois de mars 1979 apparaît la XJ « Série 3 » à la carrosserie retouchée ; elle ne reprend pratiquement aucun embouti de l’ancienne caisse même si l’aspect général reste semblable. La liste des modifications est longue : pavillon plus fuyant, pare-brise plus incliné, déflecteurs aux portes avant supprimés, poignées encastrées, pare-chocs massifs et nouvelle calandre. D’importantes modifications sur le V12 apparaissent pendant l’été 1981, c’est le HE (High Efficiency) avec culasses à toit en pente Fireball-May. À l’automne 1982, la finition Sovereign, en 6 et 12 cylindres est désormais distribuée sous la marque Jaguar et le label Daimler ne s’applique plus qu’à la très luxueuse Vanden Plas HE. La console centrale est remaniée et les placages bois font leur réapparition sur les contre-portes de tous les modèles.
   En octobre 1986, la XJ6 « Série 3 » est remplacée par la nouvelle génération de XJ dénommée XJ 40. Quant à la XJ 12, elle continue sa carrière jusqu’en octobre 1992 sous la marque Jaguar et jusqu’en avril 1993 sous la marque Daimler. Cela marque l’extinction définitive du dernier dessin du fondateur.

   In late 1979 the XJ was facelifted again, and was known as the "Series III" (pronounced Series 3). The long-wheelbase version of the car incorporated a subtle redesign by Pininfarina, externally the most obvious changes over the SII were thicker, more incorporated rubber bumpers with decorative chrome only on the top edge, flush door handles for increased safety, 1 piece front door glass without a separate 1/4 light, the grille had only vertical vaines, the reverse lights moved from the boot plynth to the larger rear light clusters and a revised roofline with shallower glass area. There were three engine variants including the 5.3L V12, the 4.2L straight 6 and 3.4L straight 6. The larger 6-cylinder, and V12 models incorporated Bosch fuel injection (made under license by Lucas) while the smaller 6-cylinder made do with carburetors. The smaller 3.4L 6-cylinder engine was not offered in the U.S. The short wheelbase saloon and coupé had been dropped during the final years of the Series II XJ. The introduction of the Series III model also saw the option of a sunroof and cruise control for the first time on an XJ model.
   The 1979 UK model range included the Jaguar XJ6 3.4 & 4.2, XJ12 5.3, Daimler Sovereign 4.2, Daimler Vanden Plas 4.2, Daimler Double-Six 5.3 and Daimler Double-Six Vanden Plas 5.3.
   In 1981 the 5.3 V12 models received the new Michael May designed 'fireball' high compression cylinder head engines and were badged from this time onwards to 1983 as HE (High efficiency) models. Also in 1981 a top spec Vanden Plas model was introduced for the US market, a model designation still used today for the top-level XJ saloon in the US.
   In 1982 the interior of the XJ underwent a minor update. A trip computer appeared for the first time and was fitted as standard on V12 models. A new and much sought-after alloy wheel featuring numerous distinctive circular holes was also introduced, commonly known as the "pepperpot" wheel.
   In 1983 the Sovereign name was transferred from Daimler to a new top spec Jaguar model, the Jaguar Sovereign. The Vanden Plas name was also dropped at this time in the UK market, as the designation was used on top-of-the-range Rover cars in the home UK market. Daimler models became the Daimler 4.2 and Double Six and were the most luxurious XJ Series III models, being fully optioned with Vanden Plas spec interiors.
   The 1984 UK model range included the Jaguar XJ6 3.4 & 4.2, Sovereign 4.2 & 5.3, Daimler 4.2 & Double Six 5.3.
   The last Series III XJ with a six-cylinder engine was produced in 1987. Production of the Series III XJ continued until 1992 with the V12 engine. In 1992, the last 100 cars built were numbered and sold as part of a special series commemorating the end of production for Canada. These 100 cars featured the option of having a brass plaque located in the cabin. It was the original purchaser's option to have this plaque, which also gave a number to the car (such as No. 5 of 100, etc.), fitted to the glove box, to the console woodwork or not fitted at all. This brass plaque initiative did not come from Jaguar in Coventry. It was a local effort, by Jaguar Canada staff and the brass plaques were engraved locally. 132,952 Series III cars were built, 10,500 with the V12 engine.
   In total between 1968 and 1992 there were around 318,000 XJ6 and XJ12 Jaguars produced, very impressive for a large executive car.