The development of the company began on June 19, 1857, when the Dortmunder Actien-Gesellschaft für Gasbeleuchtung was founded.

[4][5] DSW21 holds stakes in a large number of subsidiaries with the business areas of public transportation, energy, telecommunications, information technology (IT), housing and urban development.

[6][1] Together with the City of Gelsenkirchen and Stadtwerke Bochum GmbH, DSW21 is thus, for example, the main shareholder of Gelsenwasser AG.

[2] From 1999 to 2004 Dortmund-Märkische Eisenbahn (DME), a subsidiary of Dortmunder Stadtwerke (74%) and Märkische Verkehrsgesellschaft (26%), operated the Dortmund-Hagen-Lüdenscheid regional rail service.

[12] The mobility division of DSW21 is a partner company in the Verkehrsverbund Rhein-Ruhr (VRR), responsible for public transport in the city of Dortmund and the surrounding area.