However, the Prussian rulers did not prove to be very popular, as during the bread-riots of 1848–49 and the ensuing political upheavals, which hit the district of Düsseldorf among the hardest, policing was done from Berlin, excluding local accountability.

Thus, the Prussian government regarded the Rhinelands as more of a colony, furnishing the bureaucracy, which was based in Düsseldorf, with civil servants that were drafted in from other regions of Prussia.

Mettmann was liberated from the National Socialist Dictatorship April 16, 1945, by a vanguard of the US Ninth Army and then became a part of the British military administration under which the Northern Rhineland was redemocratised.

In the years following Germany's loss of World War II in 1945, Mettmann saw significant population increases driven mostly by the resettlement of citizens previously living in the eastern territories that had been ceded to Poland.

A shortage of industrial workers led to several recruitment campaigns in Mediterranean countries, starting with Italy in the early 1960s, and followed by Turkey later that decade.

Later events such as the Lebanese Civil War and the Balkan Wars were also reflected in the town's makeup, which now comprises communities of Turks, Kurds, Kashubians, Old Prussians, Silesians, Poles, Greeks, Croatians, Serbs, Albanians, Bosnians and Lebanese heritage, each distinct but generally well-integrated into the Rhenish-Westphalian majority population and its traditions.