Sampson earned his B.A. in Political Science from Brooklyn College and a J.D. from Albany Law School (class of 1991). He was a member of the Board of Trustees of Albany Law School.
Sampson worked as law clerk with the Department of Environmental Conservation and as a Staff Attorney to the Legal Aid Society of New York.
Starting in 1994, he worked as an attorney for the law firm of Alter and Barbaro, Esqs.
Elected in 1996 to the New York State Senate, Sampson was the Chairman of the Senate Judiciary Committee and Deputy Majority Leader for State/Federal Relations. He was also the ranking minority member of the Senate Health Committee.
Sampson had the distinction of being the first African-American to serve as the Chairman of the Senate Judiciary Committee. He also served as Chair of the Senate Ethics Committee and the Senate Administrative Regulations Review Commission.
In 2005, Sampson made an unsuccessful bid to become District Attorney of Kings County, New York, opposing the incumbent, Charles "Joe" Hynes.
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. In December 2012, Democrats elected Andrea Stewart-Cousins as their new leader over Sampson.
On May 6, 2013, Sampson was indicted by a federal grand jury for embezzlement, obstruction of justice, and making false statements to the Federal Bureau of Investigation stemming from alleged theft of $400,000 from the sale of foreclosed homes, to which he pleaded not guilty. Following his indictment, he was removed from committee assignments, ranking positions, and banned from conferencing with senate Democrats.
On July 24, 2015, Sampson was convicted of one count of obstruction of justice and two counts of making false statements to federal agents, which are felonies, and was thereby expelled from the Senate.
On March 10, 2016, the Appellate Division of the New York Supreme Court suspended Sampson from the practice of law. Because of an appeal based on the 2016 United States Supreme Court decision that overturned the conviction of former Virginia Governor Bob McDonnell, Sampson was sentenced on January 18, 2017 to 5 years in prison.