Mr. Johnston, a former Colorado state senator, benefited from far more outside spending than his opponent, who conceded on Tuesday night.
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.
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.