The two veteran politicians, who each spent the past year battling for the Democratic nomination, appeared together by split-screen on Biden's livecast.
"Today I am asking all Americans – I'm asking every Democrat, I'm asking every independent, I'm asking a lot of Republicans – to come together in this campaign to support your candidacy which I endorse," Sanders said.
The swift endorsement marked a notable change from 2016, when Sanders lost the presidential nomination race to Hillary Clinton but delayed endorsing her candidacy, in a move seen as deeply damaging in that it exposed a divide within the party.
The 78-year-old independent leftist senator from Vermont exited the 2020 race last week after losing almost all of the last primary contests, leaving Biden as the Democratic Party's presumptive nominee.
The deadly coronavirus pandemic has forced a suspension of all in-person political campaigning, as most of the nation remains under stay-at-home orders – including Biden and Sanders, who appeared online from their homes.
Sanders on Monday signalled it was crucial to unite around Biden's campaign to defeat Trump, whom Sanders branded "the most dangerous president in the modern history of this country."
"It's imperative that all of us work together," Sanders said.
Biden vs Trump
Biden said he was "deeply grateful" for the endorsement – one of many that have come from former campaign rivals including Senator Kamala Harris, Senator Amy Klobuchar and former congressman Beto O'Rourke.
"You've put the interests of this nation and the need to beat Donald Trump above all else," he told Sanders.
"As you say, 'not me, us.'"
Biden, who at 77 is mounting his third presidential run, also appealed to Sanders supporters.
"I see you, I hear you, I understand the urgency of what it is that we have to get done in this country, and I hope you'll join us," Biden said.
Biden announced the two men were establishing several policy working groups to address issues like climate change, health care and college affordability.
Sanders has been more liberal on such issues than Biden, and has successfully brought them front and center in the campaign, especially his call for a government-managed universal health care system.
Trump's re-election campaign said in a statement that Biden has "had to adopt most of Bernie's agenda" – policies that Trump has branded socialist.
"President Trump's supporters will run through a brick wall to vote for him. Nobody is running through a brick wall for Joe Biden," Trump 2020 campaign manager Brad Parscale said in a statement. – Rappler.com