He studied jurisprudence at the University of Breslau, the University of Leipzig and the University of Würzburg from 1876 to 1884, becoming a Doctor of Laws.
Upon completion of theological and juridical studies, Michaelis entered the Prussian justice ministry in 1879; from 1885 to 1889 he served as instructor at the German Law School in Tokyo. In 1892, upon his return from Japan, he joined the Prussian administration as counselor at Trier and in 1895 at Arnsberg; three years later he was director for schools and churches in the Ministry of Culture. In 1900 Michaelis became deputy to the administrative president in Liegnitz, and two years thereafter lord lieutenant in Breslau. By 1905 Michaelis had returned to Berlin as privy councillor and four years later assumed the post of undersecretary in the Prussian Ministry of Finance.
Immediately after the outbreak of war in August 1914, Michaelis became head of a newly created war grain trade society; the following year he was named director of the imperial department for the control of the grain trade. In February 1917, Michaelis was appointed state commissioner for the national food supply, therewith apparently capping a long career in the Prussian civil service.
To almost everyone's surprise, Michaelis was selected by General Erich Ludendorff on July 14, 1917, to succeed Theobald von Bethmann Hollweg as imperial chancellor and Prussian minister president. It apparently was Ludendorff's hope that this colorless but hard-working bureaucrat could calm the troubled waters lapping at the very base of the Prussian/German constitutional system. In this the general was to be disappointed. Not familiar with the world of politics, Michaelis was unable to control the parliamentary majority, against whose will he had been appointed, and equally unable to exert any moderating influence over Ludendorff. The chancellor was forced to accept the Reichstag's "peace resolution" of July 19, 1917, which demanded an end to the war without annexations or indemnities. He did so only by attaching to it the caveat "as I understand it." Thereafter, Michaelis' fortunes ebbed rapidly. He proved unwilling to articulate a clear stand on Germany's policy vis-à-vis Belgium during the Vatican peace initiative that year. As Prussian minister president he failed to draft reform proposals of the innocuous three-class voting system as promised in the kaiser's Easter message. In October the chancellor was further compromised when he attempted to uphold Admiral Eduard von Capelle's unfounded assertion that members of the Independent Social Democratic party had instigated a mutiny in the fleet. His subsequent inability to prevent the formation of a solid Reichstag front from Center to Social Democrats clearly revealed his weakness as chancellor, and on October 31, 1917, he resigned, to be replaced one day later by Count Georg von Hertling.
Michaelis finished the war as lord lieutenant of the province of Pomerania; he quit government service in April 1919. Thereafter he devoted his time to church activities and to welfare work on behalf of students. Michaelis died in Bad Saarow in the Mark Brandenburg on July 24, 1936. He had been appointed chancellor as a compromise candidate because neither of the army's two top choices, Bernard von Biilow and Alfred von Tirpitz, was acceptable to the kaiser; as chancellor, Michaelis simply proved unable to tack an independent course between generals and parliamentary majority. His pious statement of July 1917, "with God's help I will give it a try!" fully summed up his modest political talents.