On June 15, 2009, Sampson was elevated to the leadership of the Democratic Conference of the State Senate. As Conference Chairman, he is responsible for the day to day operations and the legislative agenda of the Democratic Caucus. He serves in leadership alongside former Majority Leader and Temporary Senate President Malcolm Smith.
Elected in 1996, Sampson is the Chairman of the Senate Judiciary Committee and Deputy Majority Leader for State/Federal Relations. He is a former Ranking Minority Member of the Senate Health Committee.
Sampson has the distinction of being the first African American to serve as the Chairman of the Senate Judiciary Committee. He has also served as Chair of the Senate Ethics Committee and the Senate Administrative Regulations Review Commission. Prior to his election the New York State Senate Sampson worked with the Department of Environmental Conservation and as a Staff Attorney to the Legal Aid Society of New York.
In 2005, John Sampson made an unsuccessful bid to become District Attorney of Kings County, New York, opposing the incumbent DA, Charles "Joe" Hynes.
Since 1993, he has worked as an attorney for the law firm of Alter and Barbaro, Esqs.
Sampson holds a B.A. in Political Science from Brooklyn College and a J.D. from Albany Law School (class of 1991).