London Mayor Sadiq Khan has a lot of cleaning up to do.

Khan, who made history by becoming the city's first mayor elected to a third term, has pledged to make the River Thames swimmable.

It wasn't a top campaign issue but it's an audacious goal considering the waterway was declared biologically dead not long before his birth in the city in 1970 and flows as an open sewer of sorts when heavy rains overwhelm London's ancient plumbing system.

"It's been a difficult few months, we faced a campaign of non-stop negativity," Khan said in a speech after the results showed he had won 43.8 per cent of the vote against 33 per cent for the Conservatives' candidate, Susan Hall.

"For the last eight years, London has been swimming against the tide of a Tory government … it's time for Rishi Sunak to give the public a choice."

Taming the Thames would not be Khan’s first swim upstream. His narrative is built around overcoming the odds.

As he frequently points out, he is the son of a bus driver and a seamstress from Pakistan. He grew up in a three-bedroom public housing apartment with seven siblings in South London. He attended a rough school and went on to study law. He was a human rights lawyer before he was elected to Parliament in 2005 as a member of the centre-left Labour Party, representing the area where he grew up.

In 2016, he became the first Muslim leader of a major Western capital city, overcoming an opponent whose mayoral campaign was “at least somewhat Islamophobic,” said Patrick Diamond, a public policy professor at Queen Mary University of London.

“It was seen as an affirmation of him in terms of his status as a leading Muslim politician, but also as an affirmation of London in terms of its diversity, its liberalism, its cosmopolitanism,” Diamond said. 

“That was significant in a country which doesn’t historically have a very strong track record for having diversity in its senior politicians.”

Khan has faced subtle and overt discrimination throughout his career due to his ethnicity and religion.

Some of the sharpest barbs have come from former president Donald Trump, who has feuded with him since Khan assailed Trump's campaign pledge in 2015 to ban Muslims from entering the United States.

During a campaign rally Wednesday in Wisconsin, Trump said London and Paris were “no longer recognisable” after they “opened their doors to jihad”.

Khan, who has referred to Trump as the “poster boy for racists,” responded by saying Thursday's election was a chance to "choose hope over fear and unity over division.”

“One of the things that he does incredibly well, and I would defy anyone to disagree with this, is representing London’s different and diverse communities,” said Jack Brown, a lecturer in London studies at King's College London.

“He hasn’t got absolutely everything right, but he is kind of a bringer together of different communities.”

Khan, who was ahead of the national Labour Party in calling for a ceasefire in Gaza, has taken a lot of flak for large pro-Palestinian marches in the city since the Israel-Hamas war.

But he's also known for speaking out against antisemitism and for building bridges with Jewish leaders, Brown said.

Despite his success at the polls, Khan is not an incredibly popular mayor. He’s been blamed for a lot of problems, many of which are beyond his control.

The mayor of London doesn’t have the authority of mayors in Paris or New York because power is shared with the city’s 32 boroughs and the financial district.

Candidates Count Binface, independent Tarun Ghulati, Liberal Democrat Rob Blackie, independent Natalie Campbell, Conservative Susan Hall and Mayor of London and Labour Party candidate Sadiq Khan react on the day of the results of the London mayoral election. – Reuters

Khan has a £20 billion budget that primarily goes on transport, policing and working with councils and developers to achieve his affordable housing targets that he has fallen far short of meeting. Borough councils are responsible for schools, rubbish collection, social services and public housing.

His time in office has been overshadowed by crises: first the United Kingdom's break from the European Union that weakened London's thriving financial services industry, and then the Covid-19 pandemic, which led to a cost-of-living crisis.

He has touted measures he put in place such as freezing rail and bus fares and providing free meals for all primary school pupils among his biggest achievements.

Khan has deflected a lot of criticism by blaming his difficulties on a Conservative government that has impeded his plans.

He said a projected win by Labour in a national election later this year would change his fortunes.

“For too long we’ve had a government that appears to be anti-London, that thinks the way to level up our country, to make it more equal, is make London poorer,” Khan said. 

"And that’s cutting off its nose to spite its face.”

But Diamond said a Labour government would face the same fiscal problems as the current administration and is unlikely to suddenly make Khan's life easier.

“You can’t always play the party politics card,” Diamond said. “The general sense in London is that Sadiq Khan does that too often.

"You can blame the Conservative government once or twice, but if it’s your only message, I think people maybe get a little bit tired and switch off to some extent.”

Khan has been criticised by opponents for a rise in crime — particularly incidents involving knives. He has responded by pledging more support for programs that work with youths to prevent crime while blaming government funding cuts.

In the outer suburbs, Khan has come under fire for expanding the city’s Ultra Low Emission Zone that fines drivers of more-polluting older cars £12.50 a day. Although the policy was introduced in central London by his predecessor Boris Johnson in 2015, it has widely been attributed to Khan because of its unpopular expansion, though it only applies to a small fraction of vehicles.

His main opponent Hall, a London Assembly member, vowed to “stop the war on motorists” and scrap the program on her first day in office if elected.

Khan, who has made cleaning up London's air pollution a personal mission since he developed asthma as an adult, considers those efforts among his biggest wins.

Making the Thames swimmable in the next decade would expand his mission from clean air to clean water.

Brown said that might be a more tangible achievement – given that air pollution is often invisible – but it's probably not something that won over a lot of voters.

“I don’t think many Londoners are calling out for a dip in the Thames, but why not?" Brown said. 

Labour's new Birmingham Mayor Richard Parker. – Reuters

While Khan's re-election was widely expected, Labour also snatched a surprise, narrow victory in the central West Midlands region that is home to Britain's second-largest city of Birmingham.

Conservative West Midlands Mayor Andy Street lost to his Labour opponent Richard Parker. Street's 37.5 per cent of the vote was eclipsed by 37.8 per cent for Parker, a razor-thin margin translating to 1508 votes.

Street, who has served as mayor since 2017, ran a campaign emphasising his personal record on investment while downplaying his Conservative affiliation. He publicly disputed Sunak's decision to scrap the high-speed HS2 rail link from Birmingham to Manchester last year.

Parker had sought to link him to the unpopular national government.

"I believe a Labour mayor working with a Labour government will help get Britain's future back," Parker said in a speech following the result.

Starmer said the result was beyond Labour's expectations.

"People across the country have had enough of Conservative chaos and decline and voted for change with Labour," he said.

In one bright spot for Conservatives, Ben Houchen won re-election as mayor of Tees Valley in northern England on Friday.