New York City’s largest union endorsed Maya Wiley, the former MSNBC analyst and legal counsel to Mayor Bill de Blasio, in the race for mayor on Friday, giving a lift to her campaign as she tries to prove that she is a leading candidate in the crowded Democratic field.

The powerful union, Local 1199 of the Service Employees International Union, provided one of the first big labor endorsements in the wide-open mayor’s race and hoped to use its political weight to help elect a Black woman as mayor for the first time.

The endorsement was a major win for Ms. Wiley, who is running as a progressive who wants to lead New Yorkers out of the pandemic in a city that has elected only one Black mayor and no women.

For Ms. Wiley, who did not qualify for public matching funds this week despite having announced that her campaign had met the threshold, the union’s support comes at a critical time. The union was a key early endorser for Mr. de Blasio in 2013, helping him demonstrate that he was a viable candidate in a similarly competitive race.

“Maya Wiley has the experience and vision needed to move us forward, and to reimagine what our city can be when working people have access to the tools and support needed to live with dignity,” the union’s president, George Gresham, said in a statement.

Local 1199 represents more than 200,000 health care workers, many of whom are women of color and essential workers who have worked through the pandemic. Union leaders promised to mobilize members and to use their grass-roots organization to turn out voters.

The endorsement will help Ms. Wiley make the argument that she can win the June 22 primary against other top Democratic candidates, including Eric Adams, the Brooklyn borough president, Scott M. Stringer, the city comptroller, and Andrew Yang, the former presidential hopeful. The campaign has moved mostly online during the pandemic, and it has become difficult for candidates to stand out.

Ms. Wiley faced a setback earlier this week when the city’s campaign finance board said she had not qualified for the city’s 8-to-1 matching program, which could enable her to receive $2 million in public funds.

“There were minor issues with some donations that should be resolved quickly, and the delay will not impact the campaign’s operations,” Julia Savel, a spokeswoman for Ms. Wiley, said in a statement.

A candidate must raise at least $250,000 in contributions of $250 or less from at least 1,000 city residents to qualify for the program. Then a $10 contribution from a city resident effectively turns into $90.

The campaign finance board audits each candidate’s filings. If a donor does not provide information like an address or employer, the donation is not eligible. The campaign can provide additional documentation to be considered for the next payment on March 15.

So far, only Mr. Adams and Mr. Stringer have received public funds. Mr. Yang’s campaign said he recently met the threshold and expects to receive public funding in April, pending the board’s audit.

Ms. Wiley’s campaign had made a big fund-raising push and declared victory in January, writing on Twitter: “You did it! You made history.”

The endorsement from the health care workers’ union gives Ms. Wiley’s campaign fresh energy, but some political strategists expected her to secure it. Ms. Wiley is being advised by Patrick Gaspard, the former political director at 1199 who also worked for President Barack Obama in the White House.

Gabby Seay, the union’s political director, said that she was excited to support a Black woman who has first-hand experience with racism and misogyny, especially after Kamala Harris’s history-making election as vice president.

“This is what we mean when we say support Black women,” she said in an interview.

Mr. Stringer, who is competing with Ms. Wiley for progressive voters, won an endorsement last year from the Retail, Wholesale and Department Store Union, a major union that endorsed Christine Quinn, the City Council speaker, in 2013.

Three other coveted unions have not yet made an endorsement: Local 32BJ of S.E.I.U., which represents building cleaners and airport workers, the Hotel Trades Council and the United Federation of Teachers.

Ms. Wiley thanked the Local 1199’s members for helping New Yorkers during the “darkest days” of the pandemic.

“As mayor, in my City Hall, the voices of frontline workers and unions will be as loud and as powerful as the pots and pans celebrating these essential workers at 7 p.m. every night this past spring,” Ms. Wiley said in a statement.

Ms. Wiley, a civil rights lawyer whose father was a well-known civil rights leader, worked as Mr. de Blasio’s legal counsel for two years and then led the city’s police oversight agency, the Civilian Complaint Review Board.

She has positioned herself as an expert on police reform and recently released an ambitious plan to shift resources from the Police Department to help families pay for child care and care for older relatives. Her plan aims to cut 2,250 police officers to give “high-need” families $5,000 a year to pay for those services.

Some political consultants have argued that unions have less power than they once did or that labor endorsements could matter less this year. Ms. Seay rejected that idea, saying that the union offered “boots on the ground” and noting that its members are loyal Democratic voters.

“There is no union like 1199,” she said. “Ask Bill de Blasio in 2013.”

The union interviewed eight mayoral candidates and asked them to “walk a day” in members’ shoes to see what it is like to be a health care worker. Ms. Wiley spent time with Sandra Diaz, a home health aide who later said she believed Ms. Wiley would have her back. Ms. Wiley talked about caring for her own aging mother, who had Alzheimer’s, before she died.

“She’s very down to earth, and she’s open to our ideas,” Ms. Diaz said.