Mr Lucas said: "It's now time for me to pass Star Wars on to a new generation of film-makers."
In a statement announcing the purchase, Disney said it planned to release a new Star Wars film, episode seven, in 2015.
That will be followed by episodes eight and nine and then one new movie every two or three years, the company said.
The last Star Wars film was 2005's Revenge of the Sith, and Disney said it believed there was "substantial pent-up demand".
Disney will pay about half in cash and half in stock, issuing 40 million Disney shares in the transaction.
"Our valuation of Lucasfilm is roughly comparable to the value we placed on Marvel when we announced that acquisition in 2009," Disney said, adding that the valuation was almost entirely driven by the Star Wars franchise.
The first Star Wars film was released in 1977.
"For the past 35 years, one of my greatest pleasures has been to see Star Wars passed from one generation to the next," said George Lucas.
"I've always believed that Star Wars could live beyond me, and I thought it was important to set up the transition during my lifetime."
http://www.bbc.co.uk/news/business-20146942
