Mike Johnston, a former Colorado state senator buoyed by millions of dollars in outside spending, declared victory on Tuesday night in Denver’s mayoral election, beating out a candidate who had been vying to become the first woman to hold the office.

As of 10 p.m., Mr. Johnston had pulled ahead with about 54 percent of the vote in the runoff contest, which is nonpartisan though both candidates are Democrats. His opponent, Kelly Brough, a former head of the Denver Metro Chamber of Commerce, had about 46 percent. Ms. Brough conceded minutes later.

Tens of thousands of ballots were still left to be counted as of Tuesday night, and the city said that the official results would not be released until later this month. But Mr. Johnston continued to hold a steady lead throughout the evening after polls closed.

“We can build a city that is big enough keep all of us safe, to house all of us, to support all of us,” Mr. Johnston said in a victory speech on Tuesday night. “That is our dream of Denver.”

Mr. Johnston, 48, and Ms. Brough, 59, had emerged as the top contenders from a crowded field of 16 candidates in the April general election to replace Mayor Michael B. Hancock, a Democrat who has been in office for 12 years. Term limits prevented him from running again.

Both candidates had said that they wanted to make space for more affordable housing, invest in services for homeless people and improve diversity in police recruitment in Colorado’s capital city. But Mr. Johnston appeared to have more support from left-leaning voters, and from wealthy donors outside the state.

Ms. Brough said in a speech that she had called Mr. Johnston to concede. “We set out to restore the promise of Denver,” she said. “And I still believe in this campaign, and the work we did.”

A handful of more progressive candidates had considerable support from voters before the April 4 election, but they seemed to sap each other’s momentum as the first round of voting neared.

Tami Matthews, 53, a marketing director, said she had voted for Mr. Johnston in the runoff because he seemed like a creative politician who was more progressive than Ms. Brough. She said she liked his support for more regulations to address climate change, as well as his plans to build small communities of tiny homes for the city’s homeless population.

Denver voters had been excited about the idea of having a woman as mayor, Ms. Matthews said. “But I think that there were so many other better women candidates,” she added, mentioning progressive candidates who hadn’t made it past the April election.

Still, she said she had voted for Mr. Johnston both times, even though she did not like the reports of his donations from out of state. “That does give me some heartburn,” Ms. Matthews said.

In the 12 years under Mr. Hancock’s administration, Denver has seen major population growth — despite some losses during the coronavirus pandemic — and many of the challenges that come with it. Housing costs have risen, and homelessness has gotten worse.

“Whoever wins I think will have, at least for a while, a fair mandate to make some pretty significant public policy shifts on these issues,” Seth Masket, the director of the Center on American Politics at the University of Denver, said in an interview before the early results were in. “I think a lot of other mayors of similar-size cities will be looking at Denver, just to see what comes out of this.”

Mr. Johnston — a former teacher, principal and education adviser to President Barack Obama — was first elected to the State Senate in 2009 and served until 2017, when he reached his term limit. Since then, he has run unsuccessfully both for governor and for the United States Senate.

More recently, he was the chief executive of Gary Community Ventures, an organization that combines philanthropy, investing and political funding. There, he played a leading role in advancing Proposition 123, an initiative to dedicate hundreds of millions of dollars annually to providing affordable housing. It was approved by Colorado voters last year.

Before serving as the chief executive of the city Chamber of Commerce, Ms. Brough was Denver’s head of human resources, and she served as chief of staff for former Mayor John Hickenlooper, who is now a U.S. senator.

She was endorsed by Democratic leaders in the city and state, as well as the city’s police union and its Republican Party. But endorsements from some of the progressive candidates who were edged out of the April election bolstered Mr. Johnston’s chances in the runoff.

His campaign also benefited from far more outside spending than did Ms. Brough’s, public records show.

Advancing Denver, a super PAC that supported Mr. Johnston but was not formally affiliated with his campaign, received more than $2 million from donors including Reid Hoffman, the co-founder of LinkedIn, and Michael Bloomberg, the billionaire former mayor of New York City.

Both men also supported Mr. Johnston’s unsuccessful run for governor in 2018, The Colorado Sun reported.

A Better Denver, a super PAC that supported Ms. Brough’s candidacy, was funded by donors including the National Association of Realtors, which spent more than $400,000.

Although Tuesday was Election Day, votes for the runoff contest have been rolling in since last month. That is because registered voters in Denver receive their ballots in the mail, giving them the option to send it back, drop it off or show up in person to cast a ballot on Election Day. Ms. Brough dropped off her ballot last week, and Mr. Johnston submitted his on Sunday.