Sadiq Khan, the two-term center-left mayor of London, on Saturday became the first three-time winner of the job by a comfortable margin delivering a new setback to Britain’s governing Conservative Party ahead of a looming general election.

Mr. Khan, from the main opposition Labour Party, was initially elected to the post in 2016, becoming London’s first Muslim mayor, and is now the first politician to win three consecutive terms since the role was created in 2000.

With the Labour Party well ahead in the opinion polls ahead of a looming general election, many analysts had expected Mr. Khan to cruise to a comfortable victory in a city that tends to lean to the left, but some saw the potential for an unexpectedly tight race against Susan Hall, representing the Conservatives.

That prospect quickly faded on Saturday, as it became clear that victory was looming for Mr. Khan and, in the final results, he won more than one million votes and 43 percent of the total, with Ms. Hall securing around 32 percent.

“We faced a campaign of nonstop negativity,” Mr. Khan said in an acceptance speech initially disrupted by heckling, adding, “We answered fearmongering with facts, hate with hope and attempts to divide with efforts to unite.”

The vote itself took place on Thursday along with other local and mayoral elections in which the Conservatives, led by Britain’s embattled prime minister, Rishi Sunak, suffered a series of setbacks.

The electoral system for London’s mayor has changed since Mr. Khan was last re-elected, in 2021, and the government has also introduced a new requirement for voters to produce photo ID. Some analysts feared that might deter poorer and younger voters among whom Labour tends to poll well.

Amid a squeeze on living standards, and with limited powers as London mayor, Mr. Khan had to battle to convince Londoners that he was improving their lives. Opinion polls before the vote gave him a strong lead over his Conservative rival, Ms. Hall, but a smaller advantage than his party enjoys in national surveys.

But ultimately, Mr. Khan improved on his 2021 performance after promising free school meals for pupils, a freeze on travel fares and more building of homes.

Ms. Hall had campaigned to reduce the area covered by London’s Ultra Low Emission Zone, or ULEZ, an antipollution measure that charges the owners of some older vehicles 12 pounds and 50 pence, about $15.50, for every day they drive.

ULEZ was introduced in central London by the former prime minister, Boris Johnson, when he was mayor. But it was Mr. Khan who expanded its scope to outer London, arguing that it was vital to improve poor air quality, which is known to have contributed to at least one death in London.

While inner London is a stronghold of the Labour Party, the Conservative Party normally polls significantly better in the more suburban areas of outer London, where a much larger proportion of households own cars. Last year when Mr. Johnson quit Parliament, the Conservatives won a special parliamentary election to replace him in Uxbridge, the district he had represented in outer London, after campaigning against ULEZ.

The backlash from owners of older vehicles in the area prompted a wider rethink within government over the cost of environmental policies. Not long after the Uxbridge contest, Mr. Sunak announced a weakening of Britain’s climate change targets.

In her campaign, Ms. Hall also targeted Mr. Khan’s record on fighting crime in the capital, although one of her party’s attack ads, which showed people running to safety, attracted ridicule when it emerged that the images used were filmed not in London, but at Penn Station in New York in 2017.

After finding her wallet missing last year, Ms. Hall told the radio station LBC that she thought it had been taken from her pocket on a London Underground train, using the episode as an example of how crime was out of control under Mr. Khan. The wallet was later returned by a retired businessman, who said that he had found it on a train seat and that it appeared to have been lost rather than stolen and discarded.

Ms. Hall also faced criticism after previously suggesting that the Notting Hill carnival, a famous annual Caribbean street event in West London, should potentially be relocated in the interests of public safety, and liking a social media post describing Mr. Khan as “the nipple-high mayor of Londonistan.”

Mr. Khan was on the receiving end of a more directly anti-Muslim attack from Lee Anderson, a lawmaker who was suspended from the Conservative parliamentary party after he claimed that Islamists had control of London because Mr. Khan had “given our capital city away to his mates.”

Mr. Anderson admitted his remarks were a “little bit clumsy” but refused to apologize, and he later joined Reform U.K., a small right-wing party.

But it was former President Donald J. Trump who became the London mayor’s best known critic, having feuded with him since 2016 on issues including immigration and terrorism. In 2019, after the mayor publicly opposed his state visit to Britain, Mr. Trump accused Mr. Khan of being “nasty” to him, while misspelling his name and mocking his stature.

Soon afterward, Mr. Trump also called the London mayor “a disaster,” citing several stabbings in Britain’s capital, and writing on social media that London needed to replace Mr. Khan as soon as possible.

Given that Mr. Trump is not popular in Britain, the former president’s attacks are unlikely to have damaged Mr. Khan, who disproved one of the charges leveled against him. In 2019, Mr. Trump described the London mayor as a “stone cold loser.”