The Democrat defeated his Republican rival Donald Trump in the November election after projections showed he had surpassed to threshold of 270 Electoral College votes needed to win. California, the most populous state, delivered its 55 electoral votes to Mr Biden on Monday afternoon – which officially confirmed he had got over the line.
Electors in the key battleground states of Arizona, Georgia, Michigan, Nevada, Pennsylvania and Wisconsin also confirmed a victory for Mr Biden.
The confirmation hands the former Vice-President 306 Electoral College votes to Mr Trump’s 232.
Under the US voting system, the American people cast their ballot for “electors”, who then formally vote for candidates weeks after the election.
It comes as attorney general William Barr said he would resign next week.
His departure was announced on Twitter by Mr Trump moments after counting in the Electoral College put President-elect Biden over the required 270 votes.
The president wrote: “Just had a very nice meeting with Attorney General Bill Barr at the White House. Our relationship has been a very good one, he has done an outstanding job! As per letter, Bill will be leaving just before Christmas to spend the holidays with his family.
“Deputy Attorney General Jeff Rosen, an outstanding person, will become Acting Attorney General. Highly respected Richard Donoghue will be taking over the duties of Deputy Attorney General.
“Thank you to all!”
This is a developing news story, more to follow.
Additional reporting by Luke Hawker