Joe Biden is projected to win the election and become the 46th President of the United States.
After a nail-biting race, the Democratic candidate gained an unbeatable lead in Pennsylvania, taking its 20 Electoral College votes and past the 270 needed to win the White House.
Kamala Harris will now become the first female US Vice-President in history.
Mr Biden said: "America, I’m honored that you have chosen me to lead our great country.
"The work ahead of us will be hard, but I promise you this: I will be a President for all Americans — whether you voted for me or not.
"I will keep the faith that you have placed in me."
America, I’m honored that you have chosen me to lead our great country.
The work ahead of us will be hard, but I promise you this: I will be a President for all Americans — whether you voted for me or not.
I will keep the faith that you have placed in me. pic.twitter.com/moA9qhmjn8
— Joe Biden (@JoeBiden) November 7, 2020
The President-elect accepted victory and announced in his first statement that "democracy beats deep in the heart of America".
He said: "With the campaign over, it's time to put the anger and the harsh rhetoric behind us and come together as a nation.
"It's time for America to unite. And to heal. We are the United States of America. And there's nothing we can't do, if we do it together."
The outcome makes Donald Trump the first one-term president since 1992, when George HW Bush failed to clinch four more years.
In a written statement released by his campaign, Mr Trump accused his opponent of "rushing to falsely pose as the winner".
He said: "Beginning Monday, our campaign will start prosecuting our case in court to ensure election laws are fully upheld and the rightful winner is seated.
"I will not rest until the American People have the honest vote count they deserve and that Democracy demands."
Mr Biden's path to power looked in doubt when he failed to make early, convincing gains in areas like Ohio, Texas and Iowa.
And while counting continued, Mr Trump made incendiary claims about ballot stuffing and accused his opponents of "fraud" - without providing any evidence.
He even tried to claim victory early, declaring in an address to the nation from the White House: "We will win this and as far as I'm concerned we already have won it."
— Kamala Harris (@KamalaHarris) November 7, 2020
But Mr Biden, who will be the oldest sitting president ever, urged supporters to "keep the faith" and wait until all votes had been counted.
As the hours wore on, Mr Biden finally started gaining ground - and flipped the first red state to blue in Wisconsin.
With its 10 Electoral College votes, that put him closer to the 270 needed to win.
Earlier, Mr Biden said he was on course to win a "clear majority" in the election and that as counting continued, he had already got a "mandate for action".
'A friend' to Ireland
Taoiseach Micheál Martin was one of the first to congratulate Mr Biden, saying that he has been "a true friend of this nation throughout his life".
Mr Martin added: "I look forward to working with him in the years ahead. I also look forward to welcoming him back home when the circumstances allow!"
I want to congratulate the new President Elect of the USA @JoeBiden Joe Biden has been a true friend of this nation throughout his life and I look forward to working with him in the years ahead. I also look forward to welcoming him back home when the circumstances allow! 🇮🇪 🇺🇸
— Micheál Martin (@MichealMartinTD) November 7, 2020
President Michael D Higgins also congratulated Mr Biden and Kamala Harris on the win.
He said: “Today, I have conveyed my best wishes to President-elect Joe Biden, wishing him and the American people every good fortune for his term in office and congratulating him, and congratulating Vice President-elect Kamala Harris on her historic achievement.
"The bonds between the peoples of Ireland and the United States are close and strong, and I look forward to our two countries working together to rebuild the much-needed trust in the methods and institutions of national and international solidarity and cooperation.
"That solidarity is indispensable, if we are to address the urgent challenges of our time, including such diverse issues as the need to address the cause and consequences of the Covid-19 pandemic, climate change, global poverty, hunger, migration and the digitisation of our economies, all issues on which the President-elect has expressed a commitment.
"On behalf of the people of Ireland, I wish President-elect Biden and Vice President-elect Harris every success in the years ahead.”
“I have conveyed my best wishes to President-elect Joe Biden, wishing him and the American people every good fortune for his term in office and congratulating ...Vice President-elect Kamala Harris on her historic achievement."
Full statement: https://t.co/YCxk7eWE47
— President of Ireland (@PresidentIRL) November 7, 2020
Additional reporting by IRN