In 1871, Carl Eberhard Weiss founded a small forging business with three employees in Siegen. The company produced all kinds of pushcarts, special carts for brickyards and quarries, and sack trucks. Around the turn of the century, customers of Maschinenfabrik und Dampfschmiede Karl Weiss included Rheinische Stahlwerke, Mannesmann, Buderus, Stumm, Krupp, and Thyssen.

Between 1914 and 1916, Maschinenfabrik Karl Weiss acquired Siegen-based machine factories Hoffmann and Oechelhäuser. In 1918, the three machine factories Weiss, Oechelhäuser and Hoffmann merged to form Siegener Maschinenbau AG. Mining and iron/steelmaking machinery, such as steam and blowing engines, metallurgical equipment, bicycles, and transmission and conveyor chains, were among the products manufactured by the company.

In 1927, with the takeover of Dahlbrucher Maschinenbau-Actiengesellschaft vormals Gebr. Klein, Siegener Maschinenbau AG expanded its product range to include rolling mill construction.

Siegener Maschinenbau AG was renamed Siemag Siegener Maschinenbau AG in 1939. Two years later, Bernhard Weiss acquired all shares in the company.

By the end of the 1960s already, it was clear that Siemag Siegener Maschinenbau AG could only continue to maintain its international competitiveness by merging with another major market player. Consequently, Siemag Siegener Maschinenbau GmbH and Schloemann AG concluded a cooperation agreement in 1972. Previously, both companies had handled joint orders as part of international consortia, and their product and service portfolios complemented each other. Based on this cooperation agreement, they merged to form Schloemann-Siemag AG in December 1973. GHH Aktienverein, which later became M.A.N., held 51 percent of the shares in the new company, and Siemag Weiss KG 49 percent. Both partners each held half of the voting capital. In 1974, Heinrich Weiss took over the helm of the amalgamated company.

The name "SMS", which was already being used as the short form for Schloemann-Siemag AG, officially became part of the company's name on November 1, 1980. From then on, the company was known as SMS Schloemann-Siemag AG.