The area of the present district of Unna was formerly part of the County of the Mark, which later belonged to Prussia.

In 1753 local government of the area was reformed, creating the first district of Hamm.

During the French occupation of the Napoleonic period it was included in the Ruhr département, but once returned to the rule of Prussia, the district of Hamm was recreated, although with several changes.

Later organizational changes, the latest of which was in 1975, added the towns of Lünen, Schwerte, Selm and Werne to form the district as it is today.

The district coat of arms combines two charges relating to the Counts of the Mark, the red and white/silver checked base (referring to the fess in the counts' arms), and the red lion of Berg.