Emmanuel Macron is to become France's next president after winning a projected 65% of the votes.
French polling agencies revealed the centrist candidate triumphed over the far-right's Marine Le Pen, who is forecast to have gained a 35% share.
The National Front leader called Mr Macron to concede defeat and congratulate him on his victory just minutes after the result came in.
Mr Macron - who at 39 will become France's youngest president - dashed Ms Le Pen's nationalist hopes with his pro-European stance and moderate policies.
In a sombre speech following his victory, the president-elect acknowledged divisions in society had driven people to "vote to the extreme" but said "a new page" of France's history was opening.
Theresa May was quick to offer her congratulations to Mr Macron.
In a phone call, the pair "briefly" discussed Brexit and the Prime Minister "reiterated that the UK wants a strong partnership", Downing Street said.
A Downing Street spokesman said on Sunday: "This evening the Prime Minister spoke to President-elect Macron to warmly congratulate him on his election victory.
"The Prime Minister said that she looked forward to working with the new president on a range of shared issues, with the UK and France's unique partnership providing a strong foundation for future co-operation.
"The leaders briefly discussed Brexit and the Prime Minister reiterated that the UK wants a strong partnership with a secure and prosperous EU once we leave.
"The Prime Minister and President-elect Macron looked forward to meeting and holding discussions at the upcoming Nato and G7 summits."
Crowds outside the Louvre in Paris began jubilant celebrations as news of the victory came in.
Former investment banker Mr Macron became economy minister in 2014, but resigned two years later to set up his own movement, En Marche!
He will take on France's top job without having previously been elected to public office.
Ms Le Pen had hoped to capitalise on the disillusionment of voters in France and ride a populist wave to the country's highest office.
She campaigned passionately on the need to restore French sovereignty and wanted to pull the country out of the EU and the euro.
President Francois Hollande, under whom Mr Macron served as a minister, said the results showed voters' support for the European Union, while prime minister Bernard Cazeneuve said it was a rejection of the "deadly project of the extreme right".
European Commission president Jean-Claude Juncker said he was "happy" that the ideas of a "strong and progressive Europe" would be preserved under Mr Macron's presidency.
US president Donald Trump tweeted: "Congratulations to Emmanuel Macron on his big win today as the next President of France. I look very much forward to working with him."
Ms Le Pen said that in spite of her loss, the "historic and massive" results had transformed her party into "the leading opposition force against the new president's plans".
Mr Hollande is expected to welcome his successor to the Elysee Palace within two weeks.
After a private meeting in which the nuclear launch codes will be handed over, Mr Hollande will depart.
In London, supporters gathered at The Square Pig pub in Holborn jumped for joy as Mr Macron topped the poll.
Earlier, thousands of French voters in the UK cast their ballots in the decisive presidential run-off.
Some two million French people live abroad, with an estimated 350,000 in the UK, the French embassy said.
Of the 250,000 said to be living in London, nearly 100,000 are registered voters.