In 1960, Kessler won a plurality in a six-way Democratic primary, with 38% of the vote, unseating incumbent fellow Democrat Patrick H. Kelly who represented the 10th Milwaukee County Assembly district (the 10th Ward of the City of Milwaukee); and went on to defeat Republican Herbert W. Smith 6,148 to 3,585. On the opening day of the 1961 legislative session, January 11, 1961, Kessler's 21st birthday, he became the youngest person, up to that time, ever to serve in the Wisconsin State Legislature. He was assigned to the standing committee on education. In 1962, rather than running for re-election, he sought the Democratic nomination for the Wisconsin Senate, District 6, coming in second to Martin J. Schreiber in a four-way primary race. Patrick Kelly reclaimed his old Assembly seat.