Educated first by private tutors he prepared for college at Mainz and in 1848 was awarded the degree of doctor of laws at the University of Giessen, after which he continued the study of law at Heidelberg.
At the threshold of a promising legal career he espoused with enthusiasm the movement for constitutionalism in Germany; the repressive politics of the existing government repelled him. His participation in the revolutionary activities forced his withdrawal from the country, and in 1853 he joined the large German colony at St. Louis.
He entered the mercantile business in St. Louis but public affairs were his chief interest, and he shortly assumed a prominent position among the Germans in Missouri. Strongly opposed to the extension of slavery he joined the Republican party in 1856 and labored unceasingly with voice and pen for the election of Lincoln. During the early and critical months of the war he and other German leaders joined with F. B. Blair, Jr. , in the successful effort to prevent the secession of Missouri. He raised funds for German regiments, and, as a humanitarian, supported hospitals for soldiers of both armies. In the years 1862-64, as a member of the legislature, he advocated immediate and uncompensated emancipation of Missouri slaves.
His ability and inclination brought him actively into journalism in 1862 when he founded Die Neue Zeit, which two years later was merged with the Westliche Post with Preetorius as editor-in-chief. In 1867 his intimate friend, Carl Schurz, became his partner, and the Post entered upon a long era of prosperity and of power.
Preetorius died at his home in St. Louis.
In national politics he remained a midly partisan Republican, although not seeking political preferment himself. In state and in local affairs he was independent and courageous. He believed whole-heartedly in democracy but was inclined occasionally to mistake the form for the substance, and to place too great emphasis upon mere mechanical change. In 1872 he identified himself with the Liberal Republicans.
At times impractical and intolerant, he lacked the spirit of compromise, but he never lost his early enthusiasm for progress and for personal liberty.
He was in the best sense a public-spirited citizen, sharing his prosperity with worthy civic enterprises and dispensing charity lavishly. His range of scholarship and breadth of view were unusual; he was an eloquent speaker, with a remarkable memory and a deep interest in politics, history, and philosophy.
In 1854 he married Magdalena Schmidt of Frankfort.