Penn, whose given name is Kalpen Modi (he went by the latter while working at the White House), served in the Office of Public Engagement as the the Liaison to Young Americans, the Arts, and Asian American and Pacific Islander communities.
But he is probably better known as Kumar Patel, the White Castle-loving stoner in the "Harold & Kumar" series, a new one of which is scheduled to be released in November.
TVLine.com reported earlier this month that Penn would also be joining the cast of "How I Met Your Mother."
He described his job at the White House to ABC's Jake Tapper in an interview on Thursday, video of which is available below:
"Our office doesn't handle policy, but we help kind of bridge the gaps between policies," he said to Tapper. "So if there's a group of young folks that are particularly concerned about an issue, and they want to bring in 10 or 12 folks, we'll put them in a room with some of our policy team and kind of link up that way."
Earlier this month, he moderated a "How To Make Change" discussion with an official from the U.S. Agency for International Development, where he took questions from Facebook, Twitter and a live audience. The White House billed the event as a discussion about "how young people can take the lead alleviating poverty and improving the health and well-being of people around the world."
Penn first became involved with the Obama campaign as a volunteer in 2007. He said on the "Today" show in 2009 that his grandparents were active in the Indian independence movement and that he grew up hearing stories from them about marching with Gandhi.
This isn't the first time Penn has quit working at the White House to return to acting. He resigned in June 2010 to film "A Very Harold & Kumar Christmas," only to return to government work in November.
"We deeply appreciate his service," Shin Inouye, a spokesman for the Obama administration said to The Washington Post earlier this month, "and wish him the best in his future endeavors."
Don't count out Penn for too long, though. TVline.com reports that Penn may be returning to help President Obama campaign in 2012.
Penn has blogged for The Huffington Post.
WATCH: ABC's Jake Tapper sits down with Kal Penn.