The apportionment of seats in the House of Representatives was based on the 1860 United States census.

This Congress was held during the Reconstruction era after the Civil War and U.S. President Abraham Lincoln's assassination.

The count below identifies party affiliations at the beginning of the first session of this Congress, and includes members from vacancies and newly admitted states, when they were first seated.

Senators were elected by the state legislatures every two years, with one-third beginning new six-year terms with each Congress.

Preceding the names in the list below are Senate class numbers, which indicate the cycle of their election.