Klose was the First Mayor (German: Erster Bürgermeister) of the Free and Hanseatic City of Hamburg from 1974 up to 1981, serving as President of the Bundesrat in 1979–80.

After joining the Social Democratic Party of Germany (SPD) in 1964, Klose became a member of the Hamburg Parliament (Hamburgische Bürgerschaft) in 1970, where he was vice chairman of his faction.

[citation needed] Only a year later, on 12 November 1974, Klose became First Mayor (Erster Bürgermeister) of the Free and Hanseatic City of Hamburg at the age of 37.

[citation needed] From 1991 to 1994 Klose served as chairman of the SPD group in the Bundestag, and in this position also leader of the opposition; at the time, he was chosen over two better-known candidates.

[3] Ahead of the 1994 elections, SPD chairman Rudolf Scharping included Klose in his shadow cabinet for the party's campaign to unseat incumbent Helmut Kohl as Chancellor.

[citation needed] In March 2013 the Congressional Study Group on Germany presented Klose with the inaugural International Statesmanship Award "for his longstanding service to strengthening the US-German relationship".