She was Minister President of Schleswig-Holstein from 1993 to 2005, the first woman to hold this position in any German state in Germany's history.
Simonis earned her high school degree (Abitur) in 1962 in Nürnberg. She then studied economics and sociology at the Universities of Erlangen-Nürnberg and Kiel, graduating with a degree in economics in 1967. She worked in different positions after 1967.
Simonis has been married to university professor Udo Simonis.
Simonis joined the SPD in 1969. She was elected to the German Bundestag in 1976. In 1992, she became a member of the legislature of Schleswig-Holstein. On May 19, 1993, she was elected Minister President of said state, after her predecessor Björn Engholm resigned from that position due to a scandal. She was the first woman in Germany to rise to this level in politics.
The state election in 1996 saw the SPD drop down to 39.8% (from 46.2% in 1992), but Simonis was able to form a coalition with the Greens. In February 2000, at the height of a CDU party financing scandal, the SPD was able to enlarge its share to 43.1%. Despite basing its 2005 election campaign on Simonis' popularity, the SPD suffered a heavy defeat due to the bad economic situation and dropped down to 38.7% (for the first time in almost twenty years behind the CDU).
On March 17, 2005 Simonis failed to be re-elected as Minister President of Schleswig-Holstein in 4 consecutive ballots by the Landtag (parliament) of Schleswig-Holstein. In the 1st ballot she received 34 votes and Peter Harry Carstensen (CDU) received 33 while 2 deputies cast empty votes. Neither Simonis nor Carstensen got the absolute majority (35 votes). In the 2nd and 3rd ballot the candidates received 34 votes each. Obviously one deputy of the alliance of SPD, Grüne (Greens) and SSW abstained. An unprecedented 4th ballot brought the same result. Simonis has since stepped down as Minister President of Schleswig-Holstein.