French President Emmanuel Macron was among the first to react to the pair's historic election victory.
"We have a lot to do to overcome today's challenges. Let's work together!," Macron tweeted after major US media networks announced the Democrats had won Pennsylvania and therefore the White House.
British Prime Minister Boris Johnson, meanwhile, congratulated Biden on his "election to the presidency" and on Harris for her "historic achievement".
"The US is our most important ally and I look forward to working closely together on our shared priorities, from climate change to trade and security," Johnson tweeted.
I congratulate @JoeBiden on his election as the next U.S. President & @KamalaHarris as Vice President. I know Joe Biden as a strong supporter of our Alliance & look forward to working closely with him. A strong #NATO is good for both North America & Europe https://t.co/Ij3rWtNH5c
— Jens Stoltenberg (@jensstoltenberg) November 7, 2020
As she congratulated Biden, Germany's Angela Merkel said that trans-Atlantic ties were "irreplaceable".
And Canadian Prime Minister Justin Trudeau said in a statement: "I look forward to working with President-Elect Biden, Vice President-Elect Harris, their administration, and the United States Congress as we tackle the world's greatest challenges together."
Trudeau has had a frequently stormy relationship with Trump, who once tweeted the Canadian leader was "very dishonest and weak" during a dispute on US tariffs.
Michael D. Higgins, the president of Ireland also tweeted a statement congratulating the new president and vice president, saying: "On behalf of the people of Ireland, I wish President-Elect Biden and Vice President-Elect Harris every success in the years ahead."
Indian Prime Minister Narendra tweeted his "heartiest congratulations" to Harris, who is of Indian origin. "Your success is pathbreaking, and a matter of immense pride not just for your chittis, but also for all Indian-Americans," he said.
"I am confident that the vibrant India-US ties will get even stronger with your support and leadership."
I warmly congratulate @JoeBiden and @KamalaHarris for their victory in the U.S. Presidential elections.
— Ursula von der Leyen (@vonderleyen) November 7, 2020
The EU and the USA are friends and allies, our citizens share the deepest of links. I look forward to working with President-elect Biden.
Meanwhile at home in the US, 2016 Democratic nominee Hillary Clinton tweeted her support: "The voters have spoken, and they have chosen Joe Biden and Kamala Harris to be our next president and vice president. It's a history-making ticket, a repudiation of Trump, and a new page for America."
Former president Barack Obama, who campaigned for Biden, took to Instagram to congratulate the pair. "I could not be prouder to congratulate our next President, Joe Biden, and our next First Lady, Jill Biden," he wrote.
"I also couldn’t be prouder to congratulate Kamala Harris and Doug Emhoff for Kamala’s groundbreaking election as our next Vice President."
Of course not all reactions to the Biden-Harris victory were enthusiastic.
Elsewhere, Iran's Ayatollah Khamenei said a Biden administration wouldn't change too much, adding the elections were "an example of the ugly face of liberal democracy in the US".
The situation in the US & what they themselves say about their elections is a spectacle! This is an example of the ugly face of liberal democracy in the US. Regardless of the outcome, one thing is absolutely clear, the definite political, civil, & moral decline of the US regime.
— Khamenei.ir (@khamenei_ir) November 7, 2020
