Also in 1945, the Düsseldorf chapter of the united workers union was founded, presided by Arnold.

The only in-parliament-party he did not let enter the government was the FDP, whose North Rhine-Westphalian branch had a particular nationalist bent.

On 7 September 1949, he was elected as the first president of the Bundesrat of Germany, the representation of German states at the federal level.

From 1950 to 1956, Arnold was elected minister-president for North Rhine-Westphalia twice again, governing with the help of conservative parties and the Free Democrats.

On 20 February 1956, the FDP switched coalition affiliation to the SPD, ending the Arnold government in North Rhine-Westphalia.