In 1991, Marsh was elected to the State Senate from the newly redistricted 16th Senate district. He had first won a fiercely competitive five-way contest for the Democratic primary nomination. He won re-election ten times, rising to become chair of the committee for Courts of Justice, and also served on the committees for Local Government, Finance, Rules, and Transportation. He worked to control easy firearm sales, as well as opposed the growing number of charter schools as undercutting public education. On July 1, 2014, Marsh announced his retirement from the senate (a part-time position).