Dempsey is the former general manager of The Columns Banquet and Conference Center in St. Charles, Missouri. He is a member of St Cletus
Catholic Church and the St. Charles
Rotary Club, among others. A former member of the St. Charles City Council. In 2000, he was first elected to the Missouri House of Representatives, winning reelection in 2002, 2004 and 2006. He is the chairman of the Special Committee on Parliamentary Procedure, and also serves as the chair of both the Ethics and Rules committees. Dempsey was elected to the Senate in a 2007 special election previously serving as Majority Floor Leader in the
Missouri House of Representatives. In January 2009, he was elected to
Majority Whip of the Senate. In January 2013, Dempsey was elected as President Pro Tem of the Missouri Senate. Dempsey resigned from his Senate seat in August 2015. He is now a partner at the Gate Way Group, a St. Louis based lobbying firm. ==Personal life==