Between 1886 and 1888, he served as an accounting clerk at Luxemburger Bergwerks- und Saarbrücker Eisenhütten AG in Burbach, where, during night shifts, he acquired practical knowledge of steelmaking at the plant's reheating, puddling, and blast furnaces.

This group of mines, smelteries, and rolling mills included the companies Aumetz-Friede, Hauts-Fourneaus Lorrains de la Paix, and Fentsche Hüttenwerke in Kneuttingen.

As a result of the 1919 Treaty of Versailles, all the coal and ore mines in the Lorraine region – and, with them, almost the entire foundation of the company's steel production – fell to France, which made a reorganization necessary.

In 1924, Klöckner acquired a majority holding in Rheinische Chamotte- und Dinas-Werke in Mehlem and a 50-percent stake in the nitrogen plant belonging to Gewerkschaft Victor.

[3] Parallel to setting up his steel group, Peter Klöckner founded Klöckner-Humboldt-Deutz AG, which produced engines, machines, and commercial vehicles.

In order to eliminate inefficiencies among these enterprises, Klöckner combined all of them in 1930 to form Humboldt-Deutzmotoren AG, in which his group held 70 percent of share capital.

In 1938, all these companies formally merged under a pool agreement to form Klöckner-Humboldt-Deutz AG (KHD), which engaged in the manufacture of engines, machines, and a broad range of vehicles, including tractors, diesel locomotives, and trucks.

[6] Employing some 43,000 people, Klöckner Werke AG – which held more than 99 percent of KHD's share capital – was the fifteenth largest company in Germany on the eve of the Second World War.

[1] Peter Klöckner was one of the first businessmen to integrate different plants into a vertically structured group that reflected technical developments in steel production at the time.

Almost none of the factories he owned were founded by him, but he ensured the survival of existing companies that were not organically integrated into a meaningful industrial context, by merging them and increasing their economic efficiency.

1931 Peter Klöckner heads a delegation of German industrialists on a trip to Russia, where Germany inks deals worth approximately two billion marks and becomes the Soviet Union's most important trading partner.

According to a press release, after Hitler's speech at the Düsseldorf Industrial Club in January 1932, Klöckner urgently warned about Nazi "experiments".