Labour hailed a "truly historic" result in Rishi Sunak's own backyard of York and North Yorkshire, where David Skaith smashed Tory Keane Duncan by almost 15,000 votes.
The region, which was electing a mayor for the first time, covers Mr Sunak's Richmond constituency and is an area Labour has historically struggled to compete in.
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The results as they come in
Speaking at Northallerton Town Football Club, Sir Keir Starmer said: "We've had a positive campaign here, and I am very, very proud to stand here as leader of the Labour Party to celebrate this historic victory.
"And it is a historic victory - these are places where we would not have usually had a Labour Party success but we've been able to create that success and persuade people to vote for us."
Sir Keir also renewed his demand that the prime minister call a general election.
The party also had successes in the North East and East Midlands mayoral votes and in the Blackpool by-election.
There was a sliver of hope for the government, with Lord Ben Houtchen holding on to his role as the mayor of Tees Valley.
Analysis: Labour's future success is less clear-cut after the local elections
Appearing alongside Lord Houchen at a victory rally, Rishi Sunak said: "I've got a message for the Labour Party too because they know that they have to win here in order to win a general election - they know that.
"They assumed that Tees Valley would stroll back to them - but it didn't."
This victory is likely to have quelled talk of rebellion among disenchanted Tory MPs who had threatened to oust the prime minister if the results proved a disaster, but it remains to be seen whether the Tories can hold on to the West Midlands mayoralty.
Of the 107 councils that held elections on Thursday, 102 have declared their full results, with the Conservatives losing more than half of the seats it has been defending so far.
Some 468 Tory councillors lost their seats as the party lost control of 12 councils.
Labour won control of eight councils as it gained 173 seats, while the Liberal Democrats gained 100 seats, the Greens 67, and Reform UK picked up two.
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However, Labour suffered setbacks in Oldham and Kirklees, where it lost control of the councils after victories for independent candidates opposing its stance on Gaza.
Sky News elections analyst Professor Michael Thrasher says although the results are bad news for the Tories, they do not put Labour on course for an overall majority in the Commons in a general election.
Further results are expected over the weekend, including key mayoral contests in London and the West Midlands.
Labour's Sadiq Khan is attempting to secure re-election in London, while Conservative Andy Street is defending his position in the West Midlands.
Rumours had swirled in London since the polls closed that Mr Khan could have suffered a shock defeat to Conservative Susan Hall, however Sky News understands both parties now believe the incumbent will remain in City Hall.
The results of those elections are expected to arrive at 10pm in London and 2.15pm in the West Midlands.
Other results still to be announced include council elections in the South and West of England where the Liberal Democrats and Greens hope to make progress.
There are also metro mayoral elections yet to declare a winner in Greater Manchester, Liverpool City, North Tyneside, South Yorkshire and West Yorkshire.

Labour's vote rises from 33% in 2019 to 35% on the current estimate, after more than half the wards have now declared.
The Liberal Democrats are on 16%, an increase of five percentage points on the 2019 election. This follows a familiar pattern where the party does better in council elections than in parliamentary elections.
Local election results: Relief for Sunak in key mayoral race
Other parties, such as the Greens, Reform, and independents, are projected to be on 22%.
This figure assumes also that votes for the nationalist parties in Scotland and Wales, places where no local elections took place, are unchanged from the previous election. The same condition applies to the 18 seats in Northern Ireland.
Assuming these changes in vote share occur uniformly across each of the newly drawn parliamentary constituencies in place for the next general election, Labour wins 294 seats and would overtake the Conservatives - but falls 32 seats short of gaining an overall majority.
The Conservatives fall from 372 seats on the new boundaries to just 242 seats, a projected loss of 130 seats. The Liberal Democrats rise from eight to 38 seats.
As is usual in such projections, there are individual constituencies where the count of local votes shows a party "winning" a constituency when the uniform swing suggests otherwise.
Two such examples are Aldershot and Plymouth Moor View, both of which fall to Labour when we aggregate local votes in wards lying within those constituencies.
Employing the same procedure, however, Labour's seat tally suffers when local votes in constituencies such as Blackburn and Oldham West were "won" by independents when actual votes are counted.
Labour easily retains these constituencies when uniform swing is considered.
The large vote for others highlights a growing tendency in local elections for some voters to support a range of smaller parties.
This year that tendency is exaggerated still further with the swing away from Labour towards independents in certain parts of the country.

In an interview with journalist Dan Wootton posted on X, the double Oscar winner said he will "no longer be speechless".
Last year, Spacey was found not guilty by a jury of nine sexual offences alleged by four men between 2001 and 2013 after a trial in London.
He also won a US civil lawsuit in October 2022, after being accused of an unwanted sexual advance at a party in 1986.
The Channel 4 documentary titled Spacey Unmasked is said to feature testimony from men "regarding events they say took place between 1976 and 2013, and relate to what they describe as unwanted sexual behaviour" from Spacey, according to an email shown during the interview.
'I wasn't in on the deal'
The two-part series, produced by Roast Beef Productions, is set to air on Monday and Tuesday next week.
"I take full responsibility for my past behaviour and my actions, but I cannot and will not take responsibility or apologise to anyone who's made up stuff about me or exaggerated stories about me," the 64-year-old US actor said.
"I've never told someone that if they give me sexual favours, then I will help them out with their career, never.
"...I've clearly hooked up with some men who thought they might get ahead in their careers by having a relationship with me.
"But there was no conversation with me, it was all part of their plan, a plan that was always destined to fail, because I wasn't in on the deal."
'Clumsy passes'
During the interview, he denied accusations of any illegal behaviour, but did say he had made "clumsy passes" at people in the past.
"Were there times when I would flirt with some of the people who were involved in those programmes who were in their 20s? Yes," he said.
"Did I ever hook up with another actor? Yes. Did I make a clumsy pass at someone who wasn't interested as it turned out? Yes.
"But I was not employing them, I was not their boss, I was often times just swimming in for an hour here or there as a well-known actor to lend support... to answer questions.
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"That may not have been the best decision, and it is not one that I would do today, but it happened.
"It wasn't illegal, and nor has it ever been alleged to have been illegal."
The Usual Suspects and House Of Cards star said he has struggled to get work after being acquitted of all criminal charges, branding his experience a "life sentence".
Spacey was one of the most recognised faces in Hollywood when allegations of sexual misconduct were made in 2017, leading streaming giant Netflix to cut ties with the actor.
Sky News has not been able to verify the latest allegations.

Worcestershire Country Cricket Club announced the death of the spin bowler on Thursday.
Baker had made 47 appearances across all formats for the club, taking 70 wickets and was playing for the second team on Wednesday, less than 24 hours before the news was announced.
Paul and Lisa Baker shared a photo of them with their son taken at a cricket ground saying Josh "achieved many of his dreams before reaching 21".
They added: "Take every opportunity to hug your parents and your children. We've taken great comfort from the many messages received so far."
Tributes poured in for the young sportsman including England white-ball captain Jos Buttler writing on Instagram: "Incredibly sad news, thoughts and prayers with all Josh's family, friends and all at Worcs."
Former England captain Michael Vaughan posted on X: "This is so sad... Thoughts with all his family and close friends", as did ex-batsman Kevin Pietersen who wrote: "Goodness me, this is heartbreaking! Horrific news of this up-and-coming young cricketer leaving the world way too soon!"
Azhar Ali, the former Pakistani cricketer, wrote on X: "Really sad to hear this shocking news. Josh was a wonderful and bright boy always had a smile on his face."
Baker's former coach Shaftab Khalid said the news was "devastating", speaking to BBC Sport.
Khalid added: "Worcestershire lost not just a very talented cricketer but also an amazing and wonderful human being. He was such a nice, humble and sweet person."
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England Women's coach Jon Lewis echoed their sentiments and said his assistant coach Alex Gidman - who was Baker's head coach at Worcestershire and oversaw his development into a first-team cricketer - was rocked by the tragedy.
"It's really shocking. I spoke to Alex and he was pretty cut up about it," Lewis said. "He gave Josh his first contract at Worcestershire, so he was quite connected to that part of cricket. Cricket is an incredibly small world and it's really sad."
Many county grounds had their flags at half-mast on Friday ahead of fixtures, with teams observing a minute's silence before play getting under way and some electing to wear black armbands.
Worcestershire invited fans to sign a book of condolence in memory of Baker and opened a space at their New Road ground to allow supporters to pay tribute.
They were "heartbroken" at the loss and asked for Baker's family to be shown "privacy and respect".
The former England under-19 international had taken three wickets against Somerset at Bromsgrove on Wednesday, with the game abandoned as a draw on Thursday, and also played in two Vitality County Championship games last month.
No further details about the circumstances of his death have been published.
Baker turned professional in 2021, taking 43 wickets in 22 first-class appearances, the last of which came against Durham less than a fortnight ago.

The move comes following a major review of more than 1,000 royal patronages.
It was carried out following the death of Elizabeth II and looked at the charities and organisations supported by the late queen, as well as Charles and Camilla.
It had been anticipated that the King may take a more minimalist approach to organisations he's taken on, and concentrate on those that tie in with the key focuses of his work; community, commonwealth, climate and culture.
While many do, he's also added more to his portfolio including becoming patron of the Royal British Legion, a role held by his mother from the moment she became queen.
The director general of the Royal British Legion, Mark Atkinson, said the King has "always supported the RBL, supported the armed forces community" and is "really aware of the welfare and financial support we provide to that community, day in day out".
He added that the patronage "really matters, because the King leads the nation in remembrance ensuring that nobody's service is forgotten".
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The King's patronages have increased from 441 to 669 while the Queen has an additional 15 and now heads 115 organisations. The late Elizabeth II had 492.
The Dog's Trust is another the King takes on. Its chief executive said: "It means so much to us. His late mother was our patron for many years and it's fantastic that the monarch is carrying on in that role.
"It brings to the forefront all the issues of dog welfare and the King has been a dog owner all his life."
The Queen takes on the patronage of the Royal Academy of Dance from her late mother-in-law, along with the presidency of the Sandringham Women's Institute. Both Camilla and Charles have become joint patrons for The Jockey Club.
An expert on royal patronages said the roles help the Royal Family as well as the charities.
Professor John Tribe, a senior lecturer in law at the University of Liverpool, said: "They help with fundraising, associated kudos and other links between the two organisations... but then of course the monarchy benefits, because it's a way of perhaps showing why the sovereign grant is justified.
"If we've got these various patronages being undertaken thoroughly and well, that's one of way demonstrating to the public that the monarchy is value for money."
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On Monday, the King and Queen will mark the first anniversary of the coronation privately.
Last week, they both met cancer patients in London for the King's first public engagement since he was diagnosed with the disease.