Midnight in Paris screenwriter Woody Allen triumphed over films including Bridesmaids and Young Adult to take the 'Original Screenplay' award, while The Descendants won the accolade for 'Adapted Screenplay' at Sunday night's ceremonies, held simultaneously in New York and Los Angeles.
Both movies are also nominated for screenwriting prizes at next week's Academy Awards.
The Help screenwriter and director Tate Taylor was given the Paul Selvin Award for special achievement, while Eric Roth (Extremely Loud and Incredibly Close, Forrest Gump) was honoured with the Laurel Award for Screen, a lifetime achievement award.
Judd Apatow was presented with the Herb Sargent Award for Comedy Excellence at the East Coast ceremony.
> 'Homeland', 'Breaking Bad', 'Modern Family' share WGA's top TV honours
The list of winners and nominees in the WGA awards movie categories:
Will Reiser, 50/50
Annie Mumolo and Kristen Wiig, Bridesmaids
Woody Allen, Midnight in Paris - WINNER!
Tom McCarthy and Joe Tiboni, Win Win
Diablo Cody, Young Adult
Alexander Payne, Nat Faxon and Jim Rash, The Descendants - WINNER!
Steven Zaillian, The Girl with the Dragon Tattoo
Tate Taylor, The Help
John Logan, Hugo
Steven Zaillian, Aaron Sorkin and Stan Chervin, Moneyball
Katie Galloway and Kelly Duane de la Vega, Better This World - WINNER!
Marshall Curry and Matthew Hamachek, If a Tree Falls: A Story of the Earth Liberation Front
Patricio Guzmán, Nostalgia for the Light
Wim Wenders, Pina
Hetty Naaijkens-Retel Helmrich, Leonard Retel Helmrich, Position Among the Stars
Manish Pandey, Senna