The Nineteenth Wisconsin Legislature convened from January 10, 1866, to April 12, 1866, 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 8, 1864.

[1] The governor of Wisconsin during this entire term was Republican Lucius Fairchild, of Dane County, serving the first year of a two-year term, having won election in the 1865 Wisconsin gubernatorial election.