In order to meet the increased demand for steel resulting from the expansion of the main plant, Metallwerk Ehrhardt & Heye AG in Düsseldorf-Rath was acquired in 1892 and incorporated into the company in 1896.

In 1901, on the initiative of Heinrich Ehrhardt, Rheinmetall took over the bankrupt Munitions- und Waffenfabrik AG in Sömmerda, expanding its product range.

By the end of the war, the workforce had increased to almost 48,000 workers and employees, including around 9,000 women.The built-up area at the main plant quadrupled during this time.

In 1925, the state holding company VIAG of the German Reich bought a majority stake in Rheinmetall, as part of a capital increase.

As part of the rearmament of the Wehrmacht, Rheinmetall increasingly developed and produced weapons and ammunition on behalf of the Reich Ministry of War from the mid-1930s onward.

A small number of employees and workers apparently remained in service, carrying out clean-up work and small-scale civilian production.

In the GDR, state-owned spinoffs of Rheinmetall manufactured office machines, moped engines, cameras, and later printers and personal computers.

Rheinmetall in Düsseldorf built typewriters, shock absorbers, lifts, tanning machines and transport and loading equipment.

In 1954, the first enquiries from German and Swiss former armaments manufacturers to the federal government, who wanted to buy the company, can be documented.

An upturn came in 1956, when the 56 per cent majority share in Rheinmetall-Borsig, which was owned by the Federal Republic of Germany, was taken over by Röchlingsche Eisen- und Stahlwerke GmbH.

Parallel to the increasing sales success and organic growth, Rheinmetall acquired around a dozen smaller mechanical engineering companies between 1958 and 1973, primarily those active in the fields of packaging and forming technology and electronics.

In 1989, due to the changed global political situation, Rheinmetall adapted its corporate strategy by diversifying into civilian industrial products.

The company strengthened its involvement in the field of ammunition production by acquiring a stake in WNC-Nitrochemie GmbH in Aschau am Inn.

In 1995, Rheinmetall expanded its expertise in medium-calibre automatic cannon systems, by acquiring a majority stake in Mauser-Werke Oberndorf Waffensysteme GmbH and increasing Pierburg's involvement in the USA.

In the same year, Rheinmetall acquired a stake in STN Atlas Elektronik in order to strengthen its expertise in defence electronics.

In 1999, Rheinmetall acquired a majority stake in Oerlikon Contraves, a supplier of combined cannon and guided missile systems for air defence, and Eurometaal Holding N.V., a manufacturer of medium calibre artillery.

To develop the new Puma infantry fighting vehicle for the Bundeswehr, Rheinmetall Landsysteme and Krauss-Maffei Wegmann set up the joint venture PSM GmbH, in which both companies hold a 50 per cent stake.

In July 2011, Rheinmetall reviewed the sustainability of the company's two-pillar strategy, with the two divisions Automotive Technology and Defence.

In the wake of the large-scale Russian invasion of Ukraine in 2022, German Chancellor Olaf Scholz gave the Zeitenwende speech during a special session of the Bundestag in February.

The associated change in German foreign and security policy included a special fund for the Bundeswehr, amounting to 100 billion euros.

In November 2022, Rheinmetall announced the purchase of Spanish ammunition manufacturer Expal, which was completed in August 2023 for 1.2 billion euros.

In October 2023, Ukrainian Prime Minister Denys Shmyhal presented Chancellor Scholz with the certificate of registration of the joint venture Rheinmetall Ukrainien Defence Industry LLC, based in Kyiv, which began operations in the same month.

It was announced in December 2023 that Rheinmetall planned to produce the Lynx Infantry Fighting Vehicle and the TPz Fuchs Armored Personnel Carrier in Ukraine.

[8] US intelligence services claim to have foiled a Russian plot to assassinate Armin Papperger (CEO and chairman of the executive board).

The Electronic Solutions division is concerned with the digitalisation of armed forces, infantry equipment, air defence and simulation.

[12] The Sensors and Actuators division provides equipment for industrial applications and e-mobility, as well as components and control systems for reducing emissions.

The Materials and Trade division supplies plain bearings and structural components and operates the global aftermarket business.

[20] In 2012, Rheinmetall Air Defence was implicated in a corruption case in India, along with arms dealer Abhishek Verma and his wife Anca Verma, as lodged by the anti-corruption agency of India, the CBI, for bribing defence officials for securing multi billion dollar weapons contracts of the Indian military establishment.

[21][22] In 2014, RAD Chairman Bodo Garbe and General Manager Gerhard Hoy were issued summons of the Indian courts.

[24] In 2024 Investigate Europe criticized that Rheinmetall has a parallel business that sells machinery and factories free from German export rules.