Mark B. Cohen is a Democratic politician from Philadelphia, Pennsylvania. Succeeding Eugene Gelfand, he has represented Pennsylvania House of Representatives, District 202 in the Pennsylvania House of Representatives since June 10, 1974, making him the most senior member in the Pennsylvania General Assembly since the December 27, 2010 death of Sen. Michael O'Pake, one of the most senior state legislators in the United States, and one of the longest serving state legislators in Pennsylvania history. He is one of those who "know Harrisburg," a reporter summarized. Unlike the Dean of the United States House of Representatives, being the most senior member of the Pennsylvania House is not considered a formal position. Cohen, in the 2013-2014 legislative session, still remains younger than many members of both the Pennsylvania House and the Pennsylvania Senate.
Cohen served as a member of the House Democratic leadership from January, 1990, through November, 2010, in the positions of Democratic Caucus Chairman and Majority Whip. No one has been elected as a Democratic Party caucus leader in the Pennsylvania House of Representatives as many times as Cohen.