Lori Lightfoot will be Chicago’s first black female mayor. She won as a political outsider in a city that has often elected insiders.

CHICAGO — Chicago became the largest American city ever to elect a black woman as its mayor as voters on Tuesday chose Lori Lightfoot, a former prosecutor, to replace Rahm Emanuel. When she takes office in May, Ms. Lightfoot also will be the city’s first openly gay mayor.

Ms. Lightfoot, who has never held elective office, easily won the race, The Associated Press reported, beating out a better-known, longtime politician and turning her outsider status into an asset in a city with a history of corruption and insider dealings. Ms. Lightfoot, 56, beat Toni Preckwinkle, a former alderman who is president of the Cook County Board and who had long been viewed as a highly formidable candidate for mayor.

For Chicago, Ms. Lightfoot’s win signaled a notable shift in the mood of voters and a rejection of an entrenched political culture that has more often rewarded insiders and dismissed unknowns. For many voters, the notion that someone with no political ties might become mayor of Chicago seemed an eye-opening counterpoint to a decades-old, often-repeated mantra about this city’s political order: “We don’t want nobody nobody sent.”

Ms. Lightfoot’s rise was unexpected only weeks ago, when 13 other candidates were vying to run the nation’s third-largest city, many of them far better known — with decades of experience in Chicago politics and with dynastic names like Daley. Ms. Lightfoot is a lawyer who has served in appointed positions, including as head of the Chicago Police Board and as a leader of a task force that issued a scathing report on relations between the Chicago police and black residents, but she was not widely known around the city until recent months.

The new mayor — Chicago’s 56th — arrives at a pivotal moment for a Democratic city that has for the past eight years been led by Mr. Emanuel, who surprised many residents when he chose not to seek re-election.

The new mayor — Chicago’s 56th — arrives at a pivotal moment for a Democratic city that has for the past eight years been led by Mr. Emanuel, who surprised many residents when he chose not to seek re-election.

In February, during the most crowded mayoral primary in city history, Ms. Lightfoot and Ms. Preckwinkle topped a far larger array of candidates of various ethnicities, races and genders, removing all the others from Tuesday’s runoff. That left political alignments few Chicagoans had seen before: Ms. Lightfoot pursuing white voters on the city’s Northwest and Southwest Sides; both women seeking black voters from a base on the South Side that had leaned toward a different black candidate in February; parts of Chicago’s North Side lakefront loaded with signs for Ms. Lightfoot.

Some residents said the history-making nature of the election — regardless of the winner — was energizing. But others seemed nonchalant, perhaps partly because Chicago has already seen such milestones. In 1983, the city picked its first black mayor, Harold Washington, in a racially charged election; a term before that, in 1979, the city chose its first female mayor, Jane Byrne.

“It’s not as shocking as it was back then, so that’s wonderful progress,” Kathy Byrne, the daughter of Jane Byrne, who died in 2014, said of the historic nature of this year’s election. “Why it’s taken so long? I don’t know.”

In many ways, the election transformed into a referendum on Chicago’s political culture, known for its miserable ranking on most measures of corruption, its political machine and its habit of keeping family dynasties in power.

Ms. Lightfoot, who was partner at a prominent law firm, Mayer Brown, portrayed her status as a political newcomer as a sign of strength, pledging to dismantle City Hall’s old political ways and to “bring in the light.”

Ms. Preckwinkle, the longtime Cook County board president, had tried to cast her experience as essential for managing a city the size of Chicago, while simultaneously reminding voters that she had — when she was first elected as an alderman years ago — regularly bucked the political establishment.

But as the election season played out, impatience over political corruption seemed to intensify among voters, in part because a major corruption scandal was unfolding at City Hall. Federal authorities have accused the City Council’s longest-serving alderman, Ed Burke, of running an old-school shakedown, even as the local news media reported that a second alderman had cooperated with the authorities, secretly recording his conversations at City Hall for months. Of the city’s 50 aldermen, 15 seats were up for runoff elections on Tuesday, but Mr. Burke, who has denied any wrongdoing, won re-election to another term in February, winning a margin large enough to avoid a runoff.

In recent weeks, the campaign for mayor had grown intense, boiling over with angry exchanges during debates and back-and-forths over ads and endorsements. In truth, though, the two candidates appeared to have relatively few major policy differences.

Both Ms. Lightfoot and Ms. Preckwinkle portrayed themselves as progressives. Both talked about bringing more equity to Chicago, ensuring that top-level education, affordable housing and investment ought to be spread more fairly across a long-divided city, not limited to white neighborhoods or to wealthy neighborhoods on the city’s North Side.

In nearly a year of campaigning for mayor, Ms. Lightfoot has promised that she would usher in a new era of accountability and transparency to City Hall. At times, she seemed to be running against Chicago politics in the public imagination: A television ad used an image of a smoke-filled room and spoke darkly of the “Chicago machine” that she said her opponents, including Ms. Preckwinkle, were connected to.

Still, her claim of outsider status was questioned by some younger activists in the city, who pointed to her jobs in the Richard M. Daley administration and as a police oversight official during Mr. Emanuel’s tenure.

A Lightfoot mayoralty would be different, she vowed. Ms. Lightfoot spoke frequently of equity and inclusion, of redistributing city funds to spread the prosperity of downtown and the North Side to neighborhoods that have been neglected.

“It’s unacceptable, the condition of our communities on the South and West Sides,” she said during one candidate forum. “The only way we are going to carve a new path for the city, to take us in a direction that our communities don’t continue to be resource-starved, is to vote for change.”

As mayor, Ms. Lightfoot would take power away from her own office by favoring an elected school board (currently, the mayor of Chicago appoints members of the school board).

She has vowed to enact significant change to the Chicago Police Department, increasing training and reducing officer misconduct.