The Twelfth Wisconsin Legislature convened from January 12, 1859, to March 21, 1859, in regular session. Senators representing odd-numbered districts were newly elected for this session and were serving the first year of a two-year term. Assembly members were elected to a one-year term. Assembly members and even-numbered senators were elected in the general election of November 2, 1858. Senators representing even-numbered districts were serving the second year of their two-year term, having been elected in the general election held on November 3, 1857.