When Kim Janey failed in September to qualify for the mayoral runoff election in Boston, effectively ending her time as the city’s top leader, her political rivals rejoiced and her supporters were dismayed. But her loss affected one group in particular: the collective of seven other Black women who are mayors of large cities. It’s currently a record number.

Black women mayors lead eight of the 100 cities with the largest populations in the United States, according to data from the Center for American Women and Politics (CAWP) at Rutgers University. Their disparate communities stretch across both coasts, the Midwest and the South, from Boston, San Francisco and Chicago to New Orleans, St. Louis and Washington, D.C. Some of their cities have large Black populations but others do not. And the women have forged a quiet fellowship because of their relative scarcity and similar experiences of managing the myriad facets of a big city as mayors in a shifting political landscape.

That these eight Black women have achieved this milestone is both remarkable and a long time in the making, say analysts of Black politics. The number of female mayors of any race in major U.S. cities has more than tripled in the last decade, from just nine in 2011 to 31 today, according to CAWP, which began tracking this data in 1997. But within that number, the rise of Black women has been particularly dramatic.

“This is the age of Black women in politics,” said David Bositis, a scholar of Black politics and a voting rights expert witness in federal and state courts. “This has been culminating for a long time.”

According to CAWP, the first Black female mayors of the 100 largest American cities — Lottie Shackelford of Little Rock, Ark., and Carrie Saxon Perry of Hartford, Conn. — were elected in 1987. Ms. Shackelford was in disbelief on her inauguration day, she recalled in a recent interview: “Is this really true? Is this happening?”

But for a long time, Ms. Shackelford and Ms. Perry were members of a lonely club. For decades, there were no more than two or three Black female mayors serving at the same time. That number only began to shift six years ago, rising to four in 2015, seven in 2018 and eight this year. And even as more Black women have won mayoral races across the country, the numbers of Latina and Asian American female mayors of major cities have continued to hover around one to three at a time.

In interviews with the current Black female mayors — Ms. Janey in Boston; Keisha Lance Bottoms in Atlanta; Muriel Bowser in Washington; London Breed in San Francisco; LaToya Cantrell in New Orleans; Tishaura Jones in St. Louis; Lori Lightfoot in Chicago; and Vi Lyles in Charlotte, N.C. — all eight women said they were heartened by their collective achievement, but had no illusions about the barriers still standing in the way of Black women in U.S. politics.

“It doesn’t mean that racism magically disappears. It doesn’t mean that sexism magically disappears,” said Ms. Janey of Boston.

Ms. Bowser in D.C. was the first of the eight to be sworn in, in 2015. Ms. Janey took her oath in March of this year and Ms. Jones assumed office in April. Six of the eight — Ms. Breed, Ms. Lyles, Ms. Jones, Ms. Lightfoot, Ms. Cantrell and Ms. Janey — are the first Black women to serve as mayors of their cities.

This breakthrough moment may be a fleeting one. In Atlanta, a city where nearly half of the population is Black, Ms. Bottoms announced earlier this year that she would not be running for a second term. Two Black candidates — Kasim Reed, a man and the city’s former mayor, and Felicia Moore, a woman and the current city council president — are leading the race to replace her in the Nov. 2 election, according to a recent Atlanta Journal-Constitution poll.

In Boston, Ms. Janey, who was appointed acting mayor earlier this year, came in fourth in the preliminary election this fall, failing to secure a spot in the runoff; the frontrunner to replace her, Michelle Wu, is an Asian American woman and a current city councilor. Even without Ms. Janey, though, the number of Black women mayors may not diminish. India Walton, a Democrat, is currently running for mayor of Buffalo; if elected, she would be the first woman — and first Black woman — to lead New York’s second-largest city.

Political experts attribute the rise in Black female mayors, and Black women in other elected positions, to a number of factors, including a changing electorate, grass roots activism and increased support from so-called gatekeepers, including political parties, major unions and other organizations that can help boost a candidate through fund-raising and endorsements.

This trend has accelerated in the last five years, Debbie Walsh, the director of CAWP, said: “There has been increased activism in recruiting and supporting women of color who are running for office, certainly on the Democratic side. More and more of these gatekeepers are engaging and seeking out Black women candidates.”

One political scientist also points to young Black women’s early exposure to civic engagement through sororities and other clubs, describing their political rise as “Black girl magic.”

“One of the things that I’m finding in my research is that the overwhelming majority of Black female mayors belong to a sorority — and they learned about activism in college because these sororities emphasize community service,” said Sharon Wright Austin, a professor of political science at the University of Florida and editor of the forthcoming book “Political Black Girl Magic: The Elections and Governance of Black Female Mayors.”

Even as more cities have elected Black women as mayors, other executive government positions — for which mayorships of major cities have traditionally been steppingstones — have remained out of reach. No Black woman has ever been elected governor or president. Only two Black women have ever been elected to the Senate and, with the election of Kamala Harris as the nation’s first Black, female and Asian American vice president, there are currently no Black female Senators in office.

Dr. Austin sees the increasing number of Black female candidates for these positions as encouraging nonetheless. “Before, it used to be that Black women didn’t run. They were the organizers and the campaign volunteers, but the men were the ones who were running for office,” she said. “But now you’re seeing Black women not only organizing campaigns and working in communities but having the confidence that they can run for office themselves.”

Dr. Austin cited Stacey Abrams, who narrowly lost the 2018 governor’s race in Georgia, as emblematic of the kinds of Black female candidates who are shifting the balance. Ms. Abrams rose to prominence after her loss thanks to her efforts to highlight voter suppression and mobilize Black voters in Georgia, and she has been credited with helping to flip the state for Democrats in the 2020 presidential election and 2021 Senate runoffs.

“You could argue that these candidates were unsuccessful because they didn’t win the election but you can’t really say that their campaigns are failures,” Dr. Austin said. “Because each time a woman runs, it’s sending a signal to other women that they can run, too.”

Some experts say that perhaps no other politician has a more direct and profound impact on people’s lives than a mayor, particularly in cities that operate under the strong-mayor model of governance used in most major American cities (including all but one of the cities — St. Louis — currently run by a Black woman). In this kind of system, mayors can hire and fire police chiefs, manage the city’s budget, enforce municipal policy, negotiate city contracts and in some cases even oversee cultural institutions and public transportation.

“Mayors are arguably the most important politicians in any American citizen’s life,” said Ravi Perry, a professor of political science at Howard University. “Everything that we actively deal with as citizens mostly is litigated and legislated at the local level.”

Once in office, however, Black female mayors recounted how they’ve often found themselves continuing to battle the same stereotypes that made it so difficult for them to secure their positions in the first place. Many of the current mayors talked about experiencing everyday bias, from coded language and leading questions about their qualifications to more outright discrimination.

Ms. Bottoms of Atlanta said she is often asked who is advising her — implying, she feels, that she is incapable of making decisions on her own. “It was not enough that I stood on my own two feet,” she said. “It had to be someone else or something else that was responsible for me.”

Women in these executive leadership positions, and particularly women of color, are often held to impossibly high standards, experts say, making it harder for them to accomplish their policy goals or win re-election. “It’s a scenario we call a glass cliff,” said Ms. Walsh, the CAWP director. “Expectations are set too high. And then, when they don’t meet them, it’s a steeper fall for those women.”

Part of the challenge for many of these leaders may also be the increasingly diverse electorates that have sent them to office, Andrea Benjamin, a professor of African and African American studies at the University of Oklahoma, explained. “Historically we know that Black mayors were first elected in majority Black cities. It took that kind of majority voting to get them in office,” she said. “You have to have a much broader appeal now, which can put you in a precarious position.”

Brought together by their mutual experiences, the women say they find solace in their bonds with each other. In moments of strength, happiness and adversity, they lean on each other.

“There’s definitely a sisterhood there,” said Ms. Jones of St. Louis, adding that seeing strong Black women leading major cities bolstered her resolve in her own campaign.

The mayors have text threads. They do group video chats and share jokes. They watch each other on T.V. and read each others’ statements, seeking lessons in leadership applicable to their own cities. Ms. Jones and Ms. Bottoms were in the same historically Black sorority, Delta Sigma Theta. Ms. Lyles even sent Ms. Bowser a baby gift.

The support system provides a private space for shared insights, both professional and personal. “I think that all of us recognize that we’re walking in the same shoes,” Ms. Lyles said.

In essence, the women lift each other up. For Ms. Bottoms, this sometimes means sending a text just to say: “Hey girl, I’m thinking about you. Keep your head up.”

Many of the mayors also said they felt a sense of responsibility that extended beyond the realm of local governance.

They know that millions of Black women and girls are watching them, seeking inspiration. When Ms. Janey of Boston takes video meetings, adults will often bring their children onto the screen — and when she acknowledges them, the children light up, she said.

Karen Weaver, the interim executive director of the African American Mayors Association and the former — and first female — mayor of Flint, Mich., summed up the inspiring effect these women can have for young people: “If you don’t see it, you don’t dream it.”