Johnson was first elected to the Nassau County Legislature in May 2000, succeeding his mother Barbara Johnson who died in March 2000.[4]
On February 6, 2007, he became the first male Democrat elected to the State Senate from North Hempstead in more than a century. He represented the 7th Senatorial District, which includes all of North Hempstead and parts of Hempstead and Oyster Bay. He chaired the Senate Investigations and Government Operations Committee.
In the November 2, 2010 general election, Mineola Mayor Jack Martins, Johnson's Republican challenger (who was also backed by the Independence and Conservative Parties), defeated Johnson by 451 votes.[5] On December 20, 2010, New York's highest court rejected Johnson's final appeal in regard to the election results.[6][7]