Incumbent President Donald Trump , bringing the country into strange and confusing territory.
The peaceful transfer of power has been a staple of US democracy since the 1800s and no presidential candidate has ever refused to concede defeat once all the votes were counted and legal challenges resolved.
Mr Biden's first tasks as president
Incoming president Joe Biden has pledged to restore a sense of normalcy to the White House but he will be tasked with uniting a deeply polarised America.
On Sunday, Mr Trump's supporters were emboldened by the president's denial of election defeat, with thousands protesting across the country from Atlanta to Phoenix.
Meanwhile, Mr Trump's critics booed, yelled obscenities, shouted “Loser! Loser!” and gave the finger to his motorcade as the president returned to the White House.
In his first appearance since winning the election, President-elect Joe Biden promised to work hard and restore unity for all Americans.
"I sought this office to restore the soul of America. To rebuild the back room of this nation, the middle class, and to make America respected around the world again," Mr Biden said on Sunday.
"It's time to put away the harsh rhetoric, lower the temperature, see each other again, listen to each other again and to make progress, we have to stop treating our opponents as our enemies."
In an earlier speech on Saturday, he called for an end to "partisan warfare" and asked for national unity.
President-elect Joe Biden and his wife, Jill, alongside vice-president elect Kamala Harris and her husband, Doug Emhoff. Source: President-elect Joe Biden and his wife, Jill, alongside vice-president elect Kamala Harris and her husband, Doug Emhoff.
"The purpose of our politics isn't to wage total and unrelenting war. It's to solve problems," Mr Biden said.
"We may be opponents, but we're not enemies. We're Americans."
He repeated this message earlier on Sunday after US networks declared victory for the Democrats.
"It’s time for America to unite. And to heal," he said.
During his first days as president, Mr Biden is Mr Trump's decision to withdraw from the World Health Organisation and issue an executive order requiring masks to be worn on federal property.
Prior to winning the election, he also announced he'll be re-joining the Paris Agreement and would reassure the US allies "we're back and you can count on us again".
Should Republicans keep control of the US Senate, they could possibly block, slow down or force changes to large parts of Mr Biden's legislative agenda, including expanding healthcare and fighting climate change.
That prospect could depend on the outcome of four undecided Senate races, including two in Georgia that will not be resolved until runoffs in January.
Recounts and legal disputes
Meanwhile, Mr Trump has launched lawsuits in five battleground states after making baseless claims of electoral fraud in multiple counties.
The Republican Party is seeking to raise $60 million to fund various legal challenges and so it's likely there will be many more to follow in the coming days.
A legal challenge in Michigan has been thrown out but there may be further legal challenges. Meanwhile, recounts could be held in states where the election count was close, such as Wisconsin.
The Supreme Court declined to rule on Mr Trump's legal challenge in Pennsylvania on election day but there may also be a new challenge in Nevada after his team claimed there were voting irregularities.
US President Donald Trump has been defeated after one term. Source: AAP
The deadline for resolving election disputes at the state level is 8 December and all state recounts and court contests over the presidential election results must be completed by this date.
On 14 December, the Electoral College votes for the president. Certificates recording the electoral vote results in each state must be received by the president of the Senate and the archivist no later than 23 December.
Final opportunity for appeal
Mr Biden will be tasked with appointing members of his administration and on 3 January a new congress will be sworn in, according to the .
The congressional count, which is announced by the vice president, is the final opportunity for a candidate to contest the results.
If a candidate appeals the count, a joint session would debate the objection before the Senate and House vote on whether to hold the objection.
The final results and votes from the Electoral College are declared on 6 January before Inauguration Day. Mr Biden is expected to finally take office on 20 January.
And, if Mr Trump refuses to evacuate the White House?
After the final result on Sunday, Mr Trump again disputed it, tweeting: "THE OBSERVERS WERE NOT ALLOWED INTO THE COUNTING ROOMS. I WON THE ELECTION".
In the past, he has refused to commit to a peaceful transition of power and signalled he'd fight the election result until the bitter end.
Earlier this month, Mr Biden's campaign spokesman Andrew Bates claimed: "The US government is perfectly capable of escorting trespassers out of the White House."
Mr Trump now has about two-and-a-half months remaining in office.
Additional reporting by AP /Reuters
