Mittermaier was a member of the Baden legislature for nearly 20 years previous to 1841, when his grief at the death of his son caused him to withdraw.

In 1848 he was president of the Frankfurt Pre-Parliament (German: Vorparlament), serving afterwards as representative of the city of Baden in the Parliament, where he advocated confederation, but opposed all extreme measures.

[3] His greatest claim to distinction lies in his extensive writings on jurisprudence, among which is a complete manual of criminal law, Das deutsche Strafverfahren, and he was an earnest advocate of reform in the German criminal procedure and in prison discipline.

The number of his published writings is very large, including many treatises on branches of law, discussions on all the important questions of his time connected with jurisprudence, and especially on trial by jury and the penal code.

Mittermaier used statistical evidence to claim that capital punishment was an ineffective deterrent, and influenced other German opponents of the death penalty such as Friedrich Noellner.