Being a Catholic, Marx took an active part in the political arm of German Catholics, the Centre Party. In 1899, he presided the Zentrums-Verein in Elberfeld; in 1908 he became chairman of the Centre Party in Düsseldorf.
From 1899 to 1918 Marx was a member of the Landtag of Prussia. From 1910 he was a member of the Reichstag, where he became a member of the executive committee of the Centre Party faction. There, he specialised in the field of school and culture politics. He was known as a calm and intercessional politician who didn't have many enemies and always worked towards a compromise.
During World War I he expressed his opinion against annexation and for a peace resolution. Thus, he was elected into the Nationalversammlung of Weimar. He supported the treaty of Versailles during the occupation of the Rhineland in 1923 because he thought that if Germany did not accept it, the Rhineland would be separated from Prussia. Marx also tried to unify the Centre Party and to support the government, using his style of politics and an appeal to Catholicism.
When the cabinet of Gustav Stresemann failed in 1923, Marx became chancellor, leading the tenth German cabinet since 1919. His first term lasted 13 months, his second term lasted 25 months. In this time, he presided over four cabinets, the first two being civic minority governments, later joined by the DNVP. His foreign minister was Gustav Stresemann, whose politics led to a toleration by the SPD. During Marx's terms, he managed to stabilise the German economy after the hyperinflation of 1923 by introducing a new currency. By the end of 1924, the state of emergency could be repealed. The cabinets led by Marx also accepted the Dawes Plan. In his second term, Germany joined the League of Nations, and Marx managed to unseat General Hans von Seeckt, who wanted to make the army a "state within the state". On the other hand, it should be noted that during his terms the Reichswehr worked secretly together with the Russian army to circumvent the Treaty of Versailles.
In 1925 Marx also became president of Prussia, and in 1926 he was minister of justice in the cabinet of his successor Hans Luther. He was a member of the Reichstag up to 1932. During the Nazi period, and after World War II, he lived in Bonn, where he died.