On Saturday, Affleck bested four directors who had all previously won the top DGA honor and gone on to win the Best Director Oscar.
It has been a particularly tough awards season for Spielberg, nominated by the DGA for the 11th time with "Lincoln" and a two-time winner for "Schindler's List" in 1994 and "Saving Private Ryan" in 1999.
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"What an incredible year for movies," said Spielberg. "Maybe I've had moments when I wished it wasn't such an incredible year."
Affleck also beat out Kathryn Bigelow, nominated for Osama bin Laden-manhunt thriller "Zero Dark Thirty," Ang Lee for his 3D adaptation of the bestselling novel "Life of Pi", and Tom Hooper, for his screen adaptation of the hit musical "Les Miserables".
"There was a point in my life where I was really down and really confused ... didn't know what was going to happen and I thought 'I could be a director'," Affleck told the high-powered Hollywood crowd on Saturday.
"I don't believe this makes me a real director, but I think I am on my way," he said.
Another young director also collected a top award on Saturday - Lena Dunham for Best Comedy Series for "Girls", the HBO show about four girls in Brooklyn and their travails over sex, work and making it in the big city.
"This is surreal, which I know is an over-used Los Angeles word," said Dunham, who often appears in the show she created wearing little or no clothes.