The blockbuster deal includes the surprise promise of a new ' Star Wars' film in 2015. Disney's chief executive, Bob Iger, said the plan was to release a new movie in the series every two to three years thereafter. The last 'Star Wars' picture was 'Revenge of the Sith' in 2005, and Lucas has in the past denied any plans for more.
"It's now time for me to pass 'Star Wars' on to a new generation of filmmakers," he said in a statement.
Lucas will become the second-largest individual holder of Disney shares, with a 2.2pc stake.
Disney will pay about half the purchase price in cash and issue about 40 million shares at closing.
"This is one of the greatest entertainment properties of all time," Iger said. Like Disney's purchases of Marvel Entertainment and Pixar studio, LucasFilm will "drive long-term value to our shareholders," he said.
Chief Financial Officer Jay Rasulo said the deal would lower Disney's earnings per share by a low single-digits percentage in fiscal 2013 and 2014. He also said Disney would repurchase all of the issued shares on the open market within the next two years, on top of planned buybacks.
This agreement marks the third time in less than seven years that Disney has signed a massive deal to take over a beloved studio or character portfolio, part of its strategy to acquire brands that can be stretched across TV, movies, theme parks and the Internet.
In early 2006, Disney struck a deal to acquire 'Toy Story' creator Pixar, and in the summer of 2009 it bought the comic book powerhouse Marvel.
"Disney already has a great portfolio and this adds one more," said Morningstar analyst Michael Corty.
"They don't have any holes, but their past deals have been additive."
Mr Iger said he and Lucas first discussed a possible sale about 18 months ago. Lucas was pondering his retirement, and Mr Iger was looking to add another well-known brand to the Disney empire. Each of the last three films in the series would have grossed $1.5bn in today's dollars at the box office, Mr Rasulo estimated.
The film's iconic characters also will boost Disney's sales of toys and other consumer products, particularly overseas, executives said. Sales of 'Star Wars' items such as Darth Vader and Yoda action figures total roughly $215m a year, he said.
In 2005, the year the last 'Star Wars' film was released, LucasFilm generated $550m in operating income.
Disney also will be able to extend the presence of the franchise at its theme parks around the globe, Mr Iger said. The company's parks already feature rides based on 'Star Wars' and 'Indiana Jones', another Lucas property. Film critics predicted excitement at the prospect of episode seven in the saga of Luke Skywalker and Darth Vader.