Q: Is a Jew allowed to serve on a jury in civil court, if called upon by the authorities?
A: If called for jury duty, it is permissible to serve on a jury that is established for dealing with all litigants of the population. Gentiles are also required to establish a legal system, although there is a dispute whether they are supposed to rule according to Torah law or according to their own system. They can also agree collectively to accept disqualified witnesses as valid. (See Responsa of Rema #10; Chasam Sofer vol. VI: likutim #14; Ha’amek She’eilah, Bereishis 2:3).
Since jurors are instructed to rule not only by the strict details of civil law, but also based on their understanding, one can also serve on a jury involving Jews, even if they should be adjudicating in beis din and not in civil court.(See, however, Mishneh Halachos 4:213.)