He spent most of his career working for the Prussian Ministry of Finance and then as director of the Agricultural Credit Institute in East Prussia.

As a vocal advocate of unrestricted submarine warfare, Kapp came into conflict with Chancellor Theobald von Bethmann Hollweg, who sought to prevent it out of fear that it would lead to America entering the war.

A pamphlet of Kapp's published in the early summer of 1916 entitled "The National Circles and the Chancellor" (Die Nationalen Kreise und der Reichskanzler) criticized German foreign and domestic policy under Bethmann Hollweg.

He became a proponent of the stab-in-the-back myth,[8] the belief that the German army had remained undefeated in the field and was stabbed in the back by Jews and communists at home.

He joined the German National People's Party (DNVP) in 1919 and participated in the anti-republican National Union (Nationale Vereinigung [de]).The members, which included General Erich Ludendorff, Colonel Max Bauer and Captain Waldemar Pabst, were the core group behind the Kapp Putsch that attempted to overthrow the Weimar Republic.

On 13 March, the Marinebrigade Ehrhardt, a large Freikorps unit, was able to take control of Berlin’s government quarter after the cabinet of Gustav Bauer and other leading officials fled the city.

It collapsed due a general strike, lack of participation by the Reichswehr and the refusal of the majority of government officials to take orders from Kapp.

After Dietrich von Jagow was sentenced to five years in prison, Kapp returned to Germany intending to claim his innocence and that the true criminals were those behind the German Revolution of 1918–1919.