Williams, 41, joined Mitsubishi in September 1999 following the restructuring of the UK management team. Previously, he had worked in the industry with Peugeot Talbot, Nissan, Mercedes-Benz and, prior to joining Mitsubishi, was managing director of Daihatsu UK.
Williams said: "The three years I have been with Mitsubishi have been a valuable and enjoyable experience but the opportunity to return to the role of managing director is too much of an opportunity to refuse."
Williams starts his new job at Kia on Monday October 7.
Williams is to be replaced by Lance Bradley, currently sales operations director. Bradley joined Mitsubishi from Ford in August 2000.
Jim Tyrrell, Mitsubishi UK managing director, said: "We are very sorry to lose Paul who has done a great job for us in the sales, marketing and retail functions. During the time working together, our sales have grown substantially. Importantly, our growing dealer network and ourselves are now operating profitably. We can not stand in his way, so we thank him for all he has done and we wish him well with his new role".
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