Grunert was born into a working-class family and was called upon as a Luftwaffenhelfer during World War II between 1944 and 1945 after graduating from high school.
After the war, Grunert trained as Neulehrer and worked as a history teacher in the district of Perleberg.
Grunert was appointed as the second ambassador of East Germany to the United States in 1978 succeeding Rolf Sieber.
[1] Grunert served until 1983 with accreditation to Canada where the GDR did not have a physical diplomatic presence.
From 1986 to 1990 Grunert was a professor at the Institute for International Relations at the German Academy of Political Science and Law in Potsdam-Babelsberg and in 1990 became president of the League for Friendship of Peoples of the GDR.