Thirty-nine state and territorial governorships as well as numerous state and local elections were also contested.

Democrats made a net gain of 41 seats in the United States House of Representatives, gaining a majority in the chamber and thereby ending the federal trifecta that the Republican Party had established in the 2016 elections. The Republican Party retained control of the United States Senate, making a net gain of two seats and defeating four Democratic incumbents in states that had voted for Trump in 2016. As a result of the 2018 elections, the 116th United States Congress became the first Congress since the 99th United States Congress (elected in 1984) in which the Democrats control the U.S. House of Representatives and the Republicans control the U.S. Senate. In the state elections, Democrats gained seven state governorships, control of at least 350 state legislative seats, and control of six state legislative chambers.

The elections marked the highest voter turnout seen in midterm elections since 1914, at 49.3%. The elections saw several electoral firsts for women, racial minorities and LGBT candidates, including the election of the first openly gay governor and the first openly bisexual U.S. senator. In various referenda, numerous states voted to expand Medicaid coverage, require voter identification, establish independent redistricting commissions, legalize marijuana, repeal felony disenfranchisement laws and enact other proposals.

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