On June 15, 2009, Sampson was elevated to the leadership of the Democratic Conference of the State Senate. As conference chairman, he was responsible for the day-to-day operations and the legislative agenda of the Democratic caucus. He served in leadership alongside former Majority Leader and Temporary Senate President Malcolm Smith. From January 2011 to December 2012, he served as Senate Minority Leader. On December 17, 2012, Sampson was ousted from his leadership post when Democrats elected Andrea Stewart-Cousins as Senate Democratic Leader.