The Eighteenth Wisconsin Legislature convened from January 11, 1865, to April 10, 1865, in regular session.

Senators representing even-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.

Senators representing odd-numbered districts were serving the second year of their two-year term, having been elected in the general election held on November 3, 1863.

[1] The governor of Wisconsin during this entire term was Republican James T. Lewis, of Columbia County, serving the second year of a two-year term, having won election in the 1863 Wisconsin gubernatorial election.