The Los Angeles County Coroner's Office confirmed to Eyewitness News that Breitbart died shortly after midnight Thursday at UCLA Medical Center. According to statements on his website, bigjournalism.com, he died of natural causes.
The conservative activist collapsed near his home in Brentwood while he was walking after a meeting with a business partner.
Breitbart used the Internet to ignite political scandal and expose what he saw as media bias. His hidden camera videos led to the downfall of the group called ACORN. He is also known for revealing the texting scandal that led to the resignation of Congressman Anthony Weiner.
He was revered by the right and reviled by the left. Republican presidential candidate Rick Santorum called the death "a huge loss."
"A lot of people don't like him because they don't know him. And he was a very outspoken guy. But once you got to know him, when you were in the room with him, you loved Andrew," said his father-in-law, actor Orson Bean.
He posted an edited video clip that led to the resignation of Agriculture Department official Shirley Sherrod. The video clip appeared to show Sherrod making a racist remark, but missing from the clip was the rest of the speech which was meant as a lesson in racial healing.
His liberal critics offered condolences. The watchdog group Media Matters said, "We've disagreed more than we've found common ground, but there was never any question of Andrew's passion for and commitment to what he believed."
He was the original co-editor of the Drudge Report and helped start the Huffington post. He also founded his own news network, acting as the publisher of several news websites including Breitbart.com, Breitbart.tv, Big Hollywood, Big Government, Big Journalism and Big Peace.
Breitbart seldom held back his opinions, especially to his critics, as illustrated in his last tweet at 11:25 p.m. Wednesday.
Bean said Breitbart had suffered heart problems a year ago, but it was nothing they considered serious. He said the family is still not sure what happened. Breitbart is survived by his wife and four children.
ABC News contributed to this report.