From 1984 until 1988 he was Research Assistant at the Karl-Marx-Universität Leipzig. In 1988 he joined the Justus von Liebig Landwirtschaftliche Produktionsgenossenschaft as administrator. In 1990 he became manager of the Agrar Gmbh, Taura. Since 1995 he is a sideline farmer.
From 1990 to 2002 Peter Jahr was Member of the Landtag of Saxony, the German state of Saxony's parliament. He left the Landtag to become a member of the German Bundestag representing the district of Döbeln - Mittweida - Meißen II.