Clemens Vonnegut, Sr., (1824-1906) was a Forty-eighter who was born in Germany but moved to Indianapolis after the 1848 revolution. Three years later he founded the Vonnegut Hardware Company. That same year he helped create the first Turnverein in Indianapolis (the Indianapolis Turngemeinde).

Vonnegut was an important civic leader. As well as being an active Turner, he was at various times the president of the Maennerchor, the vice president of the Normal College, and the president of the German-English School. He pressed for the teaching of German in public schools and, as a member of the Indianapolis school board, for the introduction of physical education. Vonnegut was also a Freethinker. He outlined the important tenets of the organization in a book he originally published anonymously called A Proposed Guide for Instruction in Morals from the Standpoint of a Freethinker for Adult Persons, Offered by a Dilettante.

Clemens Vonnegut, Sr., was the father of Bernard Vonnegut and the great-grandfather of the author Kurt Vonnegut, Jr.