Ms. Preckwinkle said she called Ms. Lightfoot to congratulate her just before 9 p.m. local time and though she was disappointed, she acknowledged the historic nature of the race. “Not long ago, two African-American women vying for this position would have been unthinkable,” she said. “Tonight is about the path forward.”

The women, both Democrats, were the two top vote-getters in the early round of voting on Feb. 26 that whittled the field of 14 candidates.

Ms. Lighfoot will take over a city grappling with problems including violence and trust in police, a falling population and massive pension liabilities.

Both candidates said addressing Chicago’s pervasive violence is a priority and would affect how they deal with the city’s struggling schools, communities and finances.

During the campaign, Ms. Lightfoot sought to link Ms. Preckwinkle with the city’s corrupt political machine, while Ms. Preckwinkle questioned Ms. Lightfoot’s previous leadership of a police-accountability task force.

Despite the vitriol of the campaign, Ms. Lighfoot struck a note of unity with Ms. Preckwinkle on Tuesday night. “Our differences are nothing compared to what we can achieve together,” she told supporters. “Now that it’s over I know that we will work together for the city that we both love.”

Ms. Lightfoot, who supports more-progressive tax policies, said one thing the city can do to improve its finances is rein in the millions it spends each year on settlements, judgments and attorney fees. She said the city must also do a better job about communicating its financial woes and needs to residents.

“We really have to demonstrate to the taxpayers that we’re not going to continue to treat them like an ATM machine with no limit,” she said.

Jaime Dominguez, a professor of political science at Northwestern University, said he was stunned by how few Chicago voters turned out on Tuesday. Officials with the Chicago Board of Election Commissioners said 32% of registered voters had cast a ballot in the runoff, with turnout largely mirroring that in February. Officials said mail-in and provisional ballots yet to be counted likely would bump that number up slightly in the coming days.

Mr. Dominguez said Ms. Preckwinkle seemed to struggle in her efforts to persuade voters that she could bring reforms related to criminal justice, housing opportunities and aldermanic privilege.

“This wave or undercurrent of antiestablishment politics was definitely prevalent,” Mr. Dominguez said. “Preckwinkle was just not able to detach herself from the establishment, from the ongoing corruption.”

Ms. Lightfoot led in the polls by a substantial margin ahead of election day, putting Ms. Preckwinkle and her supporters on the attack.

Michael Jackson, a 61-year-old teacher from the South Shore on the city’s majority-black South Side, said he voted for Ms. Lightfoot because she symbolizes change and is a “fresh voice” for the city.