The Friday File: the longest reigning monarch was France’s Louis XIV. He ruled from 1643 to 1715, 72 years and 110 days. Thai King Bhumibol Adulyadej who ruled from 1946 to 2014, a period of 70 years and 126 days follows and just beats out Prince Johann of Liechtenstein who ruled from 1858 to 1929, a period of 70 years and 91 days. Queen Elizabeth follows at roughly 68.5 years.