On 15 October 1918, shortly before the German Revolution of 1918-19, chancellor Max von Baden made him advisor to the Reichskanzler on issues of international law.

He also worked on plans to have Wilhelm II resign in favour of a relative and participated in negotiations at the Reichsamt des Innern (Interior Ministry) about a new constitution.

In 1919, as a close staff member of Foreign Minister Ulrich von Brockdorff-Rantzau he was Unterstaatssekretär and Generalkommissar of the German delegation at Versailles.

[1] Since he opposed German signature of the Treaty, Simons resigned his post (einstweiliger Ruhestand) and became managing director of the Reichsverband der deutschen Industrie (the industrialists' association).

[1] Simons, who never joined a political party, served from June 1920 to May 1921 in the Fehrenbach cabinet as Foreign Minister and was the Reich representative at the Spa Conference and the Londoner Konferenz (1921) [de].

In 1922, he founded the so-called SeSiSo-Kreis named after co-founders Hans von Seeckt, Simons and Wilhelm Solf, a salon meeting at the Hotel Kaiserhof in Berlin.

After Ebert's death in 1925, Simons temporarily served as acting head of state before the swearing in of Paul von Hindenburg.

Simons opposed a separate Reichsverwaltungsgericht (administrative court) and was highly critical of the Kartellgericht established in 1923 at the Reichswirtschaftsgericht, accusing it of inappropriate hostility towards cartels.

[1] After a conflict with president Hindenburg and the government of Hermann Müller concerning the appointment of members of the board of directors at the Reichsbahn in 1928, Simons resigned in 1929.

[1] After the Nazis seized power in 1933, Simons was active only in the church and in the Neue Bachgesellschaft honouring Johann Sebastian Bach (where he was president since 1930).