Lee Kuan Yew (Singapore)
52 years (1959 – 2011, Wikipedia)
Note from Wikipedia: After leaving office as Prime Minister, Lee was given the emeritus positions of Senior Minister (1990–2004) and Minister Mentor (2004–2011). The latter title, unique in world politics, maintained Lee’s status as a “power behind the throne” in Singaporean politics.
(Fidel Castro’s had Cuba under 52 years of dictatorship)