Mende studied law in Marburg and Göttingen and passed his state legal exams in 1987.

Thereafter Mende was employed as the president of the regional council in Kassel, at the former Federal Office for the Recognition of Refugees (now the Federal Ministry of Migration (Bundesamt für Migration), at the Hesse State Welfare Agency (Landeswohlfahrtsverband Hessen) in Kassel, at the Schleswig-Holstein State Insurance Institution (Landesversicherungsanstalt Schleswig-Holstein) and at Lower Saxony's State Ministry for Social Welfare, Women, Families and Health (Niedersächsisches Ministerium für Soziales, Frauen, Familie und Gesundheit).

From 1999 to 2003 Mende was the leader of the SPD party in the State Parliament of Lower Saxony.

From 2003 to 2008 Mende was the divisional head (Referatsleiter) of the Lower Saxon Ministry for Home Affairs, Sport and Integration.

From 22 February 2009 Mende served as the Lord Mayor (Oberbürgermeister) of the town of Celle in Lower Saxony, having won at the first vote with a clear and "surprising" majority of 50.8%.