From 1975 to 1991, Eichel served as mayor of Kassel, initially gaining the office at the age of only 33. From 1991 to 1999, he was the Minister-President of Hesse in a coalition with the Green Party. The coalition won again four years later and was the first red-green coalition to serve two consecutive terms. Eichel also served as President of the Bundesrat from November 1998 to April 1999. However, he unexpectedly lost the 1999 state elections to Roland Koch's CDU and lost his office.