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.
In 1901 the town of Hamm itself became a separate urban authority, but the district retained the name of, and its administration continued to be located in Hamm until 1929. In that year the administration was moved to Unna, when the district name also changed to Unna to reflect the new administrative centre. 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.