An intellectual activist and regional politician in the People's State of Hesse, he played a major role in the propaganda of the SPD and the anti-fascist Iron Front during the last years of the republic.

He returned to active duty in November 1917 and served on the western front; the next year, he was personally awarded the Iron Cross first class in a ceremony by Kaiser Wilhelm II.

He later recalled that, while serving on the western front in October, his unit closely followed news from Russia: "The common refrain of our evenings was that 'we want to be good Bolsheviks.'"

[1]: 123–124  Mierendorff began studying political science in Heidelberg in 1919 and, after brief moves to Munich and Freiburg, completed his dissertation on "the economic policy of the German Communist Party" in 1923.

After right-wing and völkisch students began disrupting lectures by liberals such as Max Weber and Albert Einstein, Mierendorff joined the SPD in January 1921.

[1]: 125 After the assassination of foreign minister Walther Rathenau in 1922, the government ordered all state institutions to close temporarily and fly the republican flag in his honour.

The executive considered placing him on the staff of Otto Braun or Carl Severing in the Prussian government, but Mierendorff had no desire to work under these veterans.

He believed that most active voters had accepted the republican form of government, but monarchist spirit manifested in disputes over policy, such as the use of the imperial flag and the expropriation of the princes.

[1]: 128 Mierendorff was also critical of proportional representation in the electoral system, which he blamed for Germany's fragmented political landscape and unstable coalition governments.

At this time, he believed that the Nazis could win up to sixty seats in the Reichstag in the next election, drawing support mostly from the bourgeois parties whose voters would be swayed by its emotional rhetoric and opposition to Marxism.

By 1931 he believed that Nazi success grew from many sources of social discontent, not only the economic crisis as was party orthodoxy, although he discounted antisemitism as a major factor.

He proposed that SPD propaganda focus on internationalism and reconciliation to counter Nazi nationalism and chauvinism, economic recovery to win over the disaffected, and reform of the electoral and federal system to bolster democratic legitimacy.

[1]: 131–133 In September 1931, Mierendorff and Leuschner uncovered the Boxheim Papers, a collection of internal Nazi documents which detailed their plan to deal with a left-wing uprising, involving martial law, executions, and cutting off supplies to worker-occupied cities if necessary.

The papers were publicised in November and made headlines, but no charges were brought over the issue as bourgeois leaders, who viewed the Nazis as potential allies, declined to pursue the case.

Mierendorff recruited exiled Russian social democrat Sergei Chakhotin to develop the organisation's image and propaganda, hoping to appeal to those who were not swayed by traditional means such as the press and public meetings.

They created the Three Arrows representing "unity, activity, and discipline", although they were most famously used in an SPD poster for the November 1932 election to depict the struggle against monarchism, fascism, and communism; they later became an iconic symbol of social democracy across Europe.

[1]: 133–134 Mierendorff hoped that a more activist, propaganda-focused strategy would reinvigorate the SPD and unite its divided left and right wings in pursuit of action against Nazism on the ground.

When Mierendorff and Chakhotin approached the SPD executive with their proposals ahead of the Prussian state election in April, they were faced with concerns about public image and the potential for trouble with the police.

Building on his previous criticisms, he stated that the SPD had failed by refusing to challenge nationalist foreign policy, push constitutional reform, or put forward an alternative economic vision.

Though he welcomed to adoption of a program advocating "socialist restructuring of the economy" after the July election, he believed that the party's noncommital approach to extraparliamentary action or new forms of propaganda rendered it ineffective.

[1]: 136–137 Mierendorff's faith in the SPD leadership deteriorated as they refused to consider extraparliamentary action in the midst of the deepening crisis: "There is no one interested in our agitation, and the work to reform the party from within — oh God, that, too, looks anything but encouraging."

He helped organised mass protests in Frankfurt and Darmstadt in response to the appointment of Hitler as Chancellor in January 1933, but struggled to counter the pessimism which gripped the social democratic movement.

He held out hope that the constitution would not be immediately dismantled, and that the Nazi government would fail to fulfill its economic promises, giving new strength to the forces who opposed them.