The apportionment of seats in the House of Representatives was based on the 1960 United States census.

Both chambers had a Democratic supermajority, and with the election of President Lyndon B. Johnson to his own term in office, maintaining an overall federal government trifecta.

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 are popularly elected statewide 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.