The Playboy founder has donated the final $900,000 needed to preserve 138 acres behind the film capital's most famous visual symbol on Mount Lee, just as time was running out.
"It would have been a real shame after having restored it if it wound up sold," Hefner, 84, told PEOPLE on Monday at the Hollywood premiere of Iron Man 2, with the legendary 45-foot white letters looming in the background. "It's become something iconic and represents not only the town but represents Hollywood dreams, and I think that's something worth preserving."
Developers have been eyeing the hillside, once owned by Howard Hughes, for luxury mansions, but a nonprofit land-conservation group was given the chance to buy the land for $12.5 million. As an April 30 deadline neared, the group was nearly $1 million short.
"I was aware of the fact that they were raising the money, but I only learned about a week ago that they were running out of time," said Hefner. "They only had about a week and a half left to go."
The sign was built in 1923 as part of a real-estate promotion, originally spelling out "Hollywoodland" (the "land" was removed in 1949). In 1978, after years of neglect, Hefner was one of a number of patrons who donated funds to restore it.
Meanwhile, Hefner continues to develop his own Hollywood fantasy: a feature film chronicling his life, with Iron Man star Robert Downey Jr. potentially in the lead role.
"It would be my hope and dream," said Hefner. "Right after Iron Man first came out, he called me the following week and said, 'When are we going to start the Playboy film?' So, we have to get a good screenplay."