Liberal Democrat Sarah Olney has defied the odds and Zac Goldsmith's 23,000 vote majority to beat the independent incumbent and become the new MP for Richmond Park and North Kingston.
The dramatic win comes just a day after former deputy prime minister Nick Clegg told IBTimes UK that his party was set to secure an "astonishing win" over the former Conservative MP.
The by-election was triggered after Goldsmith quit the House of Commons over the government's decision to back a third runway at Heathrow Airport, in west London.
But Brexit was just as big an issue, with the Liberal Democrats taking a pro-EU stance and highlighting Goldsmith's Euroscepticism.
The tactics, which included a last-minute endorsement from musician and campaigner, Sir Bob Geldof, paid off as Olney won 20,510 of the vote against Goldsmith's 18,638 on a turnout of 53.6%.
The result means the Liberal Democrats now have nine MPs in the House of Commons, which is expected to embolden Remain campaigners.
Liberal Democrat MP Alistair Carmichael, speaking after Olney's victory, told IBTimes UK that the voters of Richmond Park had sent a warning to Theresa May about a so called "hard Brexit".
"I get the sense that the 48% who voted to remain in the EU are now feeling completely alienated by May and her government," he said.
"There's a feeling that she does not listen to them and does not respect their views...she has duty to bring the whole back together and she is not doing that. Until she does this sort of thing is going to keep happening."
The Conservatives, who like the Greens and Ukip did not contest the by-election, commiserated Goldsmith on his defeat.
"Zac has been a strong and principled champion for the residents of Richmond Park and North Kingston over the past six years as their Member of Parliament, and a popular figure in the Conservative Party. We are sorry that he is no longer in the House of Commons," a Tory spokesperson said.
"This result doesn't change anything. The Government remains committed to leaving the European Union and triggering Article 50 by the end of March next year.
"In addition, we will continue to take decisive action in the national interest to secure the UK's place the world - supporting a third runway at Heathrow to secure jobs and business opportunities for the next decade and beyond."
Goldsmith, meanwhile, stressed the by-election was not a "political calculation" and wished Olney well in her "very, very important job".