The donation comes from Margie Petersen, the widow of the museum's founder Robert Petersen, who died in 2007 at age 80, and the Margie & Robert E. Petersen Foundation.
The donation includes an unspecified cash amount, along with more than 150 cars from Robert Petersen's personal collection and the museum's 300,000-square-foot building.
"One word I can use is speechless," said Buddy Pepp, the museum's executive director. "It is something spectacular. It will ensure the museum's existence for my great, great, great grandchildren to come."
The Petersens founded the museum in 1994 after building a publishing empire that included Hot Rod and Motor Trend magazines.
"We'll be doing such mundane things as fixing the roof, painting and replacing the carpet," Pepp said. "We'll be able to put a research library and an archive library that not only will our staff be able to use but also the public."
The museum also plans to use some of the money to give employees a raise, as wages have been frozen for more than two years.
The museum draws an estimated 150,000 visitors a year.