Elizabeth Ashford, Harris' chief of staff, said in an email that Duran had resigned "to pursue other opportunities."
"We will miss him, but the AG supports his decision and wishes him the best," she said.
Duran was Brown's press secretary before leaving the administration in February. Ashford, previously Brown's chief deputy press secretary, left Brown's office in April.
Before coming to work for Brown in 2011, Duran was communications director for U.S. Sen. Dianne Feinstein from 2008 to 2010.
In an email, Duran said he is currently in Europe.
"As Heraclitus said, change is the only constant," Duran said. "After a decade working for some of the most amazing leaders in California politics, it's time to try something new. Right now I'm traveling in Europe. When I get back to Oakland, I'll probably do some consulting and write a book."
The book, he said, will not be about politics.
Editor's note: This post was updated at 5:23 p.m. to include Duran's remarks.
PHOTO: Gil Duran, California Gov. Jerry Brown's press secretary, right, during a news conference in January. The Sacramento Bee/RandallBenton