In 1887 he began his career in civil service at the administrative district (Landkreis) office in Freienwalde, Brandenburg whose head was Theobald von Bethmann Hollweg, the later Chancellor of Germany.

During World War I, he agitated against the 1917 Reichstag Peace Resolution, advocated the use of unrestricted submarine warfare and rejected the reform of the Prussian three-class franchise initiated by Chancellor Bethmann Hollweg.

In 1925 the DNVP had temporarily abandoned its anti-republican attitude by joining the German cabinet in a liberal-conservative coalition government under Chancellor Hans Luther.

However, in the 1928 federal election, the party suffered a disastrous defeat and had to accept the formation of Social Democratic government under Chancellor Hermann Müller.

Westarp resigned as chairman; when in 1930 his support for the government of Heinrich Brüning did not meet with the approval of his party, and because of the anti-republican tendencies of his successor Alfred Hugenberg which became manifest during the 1929 referendum, he finally left the DNVP in 1930.