"Our country is being destroyed by incompetent people," said Trump, who devoted large swathes of a jovial speech to poking fun at 81-year-old Biden. "All presidents should have aptitude tests."
The former president addressed Club 47 fan club members at a convention centre in West Palm Beach, a short drive from his Mar-a-Lago residence.
As part of the festivities, organisers brought out a towering, multi-layered cake while audience members tossed red and blue balloons.
Set on a gold-coloured base, the cake featured separate tiers that included a Make America Great Again baseball cap and the Club 47 logo, an American flag, the phrase Born in the USA on Flag Day, a depiction of Trump golfing as well as Trump and Republican logos.
When Trump took the stage, the crowd sang happy birthday and chanted USA, USA.
The layer cake was just for show. Backstage, there was a sheet cake with vanilla icing that was served to some members of the presumptive Republican nominee's campaign staff.
The event, in Trump's adopted home state, sold out of 5000 tickets at about $35 apiece, with closer spots to the stage costing $60, according to Club 47 president Larry Snowden.
"This is the biggest birthday party I've ever had by far," Trump said.
Donald Trump has used a birthday celebration to again criticise President Joe Biden. (AP PHOTO)
The former president elicited strong cheers by listing his now-familiar campaign plans, including discussing immigration in menacing terms and pledging to reduce regulations, scrap environmental protections to stimulate domestic energy production and cut taxes.
Despite so often scoffing at Biden, even declaring that the president often "doesn't know where the hell he is," Trump also offered a seemingly contradictory message to his own supporters.
He endorsed early voting, casting ballots by mail and also on election day in person, only to later note: "I actually tell our people, we don't need your vote. We've got so many votes."
It was yet another strong show of support for Trump and came a day after Republicans in Congress sang their own rendition of happy birthday and presented the former president with a cake and gifts during a Thursday visit to Capitol Hill.
Even after becoming the first former president to have been convicted of a felony, Trump has tightened his grip on much of his party's base and elected officials.
Next month, he is scheduled to accept his party's presidential nomination for the third time — despite facing sentencing in his hush money case on July 11.
Trump referenced his conviction on 34 felony counts on Friday, declaring, "In the end, they're not after me. They're after you and I just happened to be standing in their way".
Biden's campaign marked Trump's birthday by compiling a listing of "78 of Trump's historic… 'accomplishments,'" with links to media coverage of policy proposals including "cutting social security and Medicare," Trump's presidency during GOP losses in the US house and Senate and several references to his legal cases.
Club 47 is based in Palm Beach County and says on its website that its goal is to keep Trump's supporters "in our area connected and engaged."
Lydia Maldonado, a local Hispanic activist, said Friday's event was unique compared to any rally nationwide with the former president and that Trump felt comfortable and familiar with the crowd.
"The purpose of having this event is pretty much to let him know how much the community here loves him and how much the community supports him," Maldonado said.
