Speaking after Chicago Mayor Rahm Emanuel, who was Obama's former Chief of Staff, Penn, who resigned last year from his White House position as associate director in the office of public engagement to return to acting and complete the sequel to his zany Harold and Kumar comedy trilogy, had the crowd cracking up when he began his remarks to the wildly sustained applause, saying, "I am honoured to accept your nomination for president of the United States."
"Wait, this isn't my speech. Prompter guy, can we pull up my speech?" he said, as the audience continued to crack up.
Penn went on to shower kudos on Obama and his achievements ranging from eliminating Osama bin Laden to preserving Pell grants and college tuition credits to supporting gay marriage.
"I've worked on a lot of fun movies," he said, "but my favourite job was having a boss who gave the order to take out bin Laden -- and who's cool with all of us getting gay-married."
Then he took a hefty swipe at the 82-year-old Clint Eastwood's rambling speech at the Republican National Convention in Tampa, Florida, last week, where the ageing actor of the Dirty Harry series spoke to an empty chair which was supposed to be an imaginary Obama. "Thank you, invisible man in the chair, for that, and for giving my friends access to affordable health insurance and doubling funding for the Pell grant," Penn said.
He recalled that he had started volunteering for Obama in 2007, "but nothing compares to what I saw behind the scenes at the White House, when I had the honour to serve for two years as President Obama's liaison to young Americans. I saw how hard he fights for us."
Penn said, "One of the most special days was a Saturday in 2010. The Senate repealed 'Don't Ask, Don't Tell,' so anyone can serve the country they love, regardless of whom they love. But that same day, the DREAM Act was blocked. That bill would give immigrant children -- who've never pledged allegiance to any flag but ours -- the chance to earn their citizenship. Simple. Important."
Continuing to reminisce, he said, "I was in a small office on the second floor of the West Wing with eight other staffers. We'd worked our hearts out and cared deeply about what this would mean for other young people. There wasn't a dry eye in the room -- five minutes later, President Obama walked in, sleeves rolled up, and he said to us, 'This is not over. We're gonna keep fighting. I'm gonna keep fighting. I need young people to keep fighting.' That's why we're here!"
Then warning of how students could be affected if the Republicans came to power, Penn said, "Our Republican friend said, 'Sure you can do that' but one of the things they were willing to trade is a little item called the college tax credit, which today is saving students up to $10,000 over four years of school."
Echoing the Obama campaign theme of reaching out to middle-class voters, particularly among the independents and that the Obamas could empathise with their plight unlike Republican presidential nominee Mitt Romney, who as a multi-millionaire was out of touch with the masses, the actor turned political activist spoke of how "President Obama paid off his own student loans not too long ago and he remembers what it is like."
Penn said Obama was convinced that "making it easier to go to college and get technical training is exactly how we grow out economy, create jobs and out-compete the world. So he stood firm. And that tuition tax credit is still here. But, if we don't register, if we don't vote, it won't be."
"I volunteered in Iowa in 2007 because, like you, I had friends serving in Iraq, friends who were looking for jobs, others who couldn't go to the doctor because they couldn't afford it. I felt that had to change. So I knocked on doors. I registered voters."
And then turning to humour while ostensibly continuing to hammer out the message to try and recapture the youth vote that according to some analysts and polls had plunged as much as 10 percent or more for Obama, he said, "I ask all you young people to join me. You don't even have to put pants on. Go to commit.barackobama.com and register right there. And the oldies out there, you can do it, too."
"I really enjoyed listening to Rahm's speech, but he's a mayor now, so he can't use four-letter words. But I am no mayor " And, as the crowd waited in what seemed like a pregnant pause if ever there was one, Penn said, "So, I've got one for you -- VOTE!"
Meanwhile, a video that was being circulated by the campaign after it was announced that Penn would host the Democratic National Convention's live stream coverage on the final day, of Obama, making a call to Penn from the Oval Office went viral online.
In the video, Obama, hoping that Penn would take on the duties of hosting the live coverage asks the actor, 'Hey, this is Barack. Listen, I need to know if you're on board. Okay, good. Because I'm counting on you. Everybody is. We have to get this right. So there's a lot at stake here.'
'Just remember that I'm trusting you on this and I'll see you then,' Obama adds at the end of which the video, which then pans to a shot of Penn lolling on a couch with John Cho, his counterpart who plays Harold in Harold and Kumar.
Cho tells Penn, 'Dude, who was that? That sounded intense,' to which Penn replies with a blank stare of a stoned individual, sans any semblance of enthusiasm, a perennial ingredient of the Harold and Kumar series, 'The president.'
'Sweet,' says Cho and moments later the two are seeing laughing uproariously at some of the cartoons they are watching on TV.