The event took place Wednesday at an untelevised gala dinner at Cipriani's in Manhattan, N.Y.
The National Board of Review had announced its winners in December. The Irishman took home Best Film, with Sandler winning Best Actor for his role in Uncut Gems and Pitt taking Best Supporting Actor for Once Upon a Time in Hollywood.
The gala followed the The Irishman's win for Best Picture at the New York Film Critics Circle awards in December.
Once Upon a Time in Hollywood also won Best Director for Quentin Tarantino, while Uncut Gems took home Best Original Screenplay for co-directors and writers Josh and Benny Safdie and Ronald Bronstein.
Bradley Cooper presented Best Supporting Actor to Pitt, who credited Cooper with his sobriety in his acceptance speech.
"I got sober because of this guy," Pitt said. "And every day's been happier since."
"I love you, and I thank you," he added.
Here's the full list of National Board of Review winners:
Best Film -- The Irishman
Best Director -- Quentin Tarantino, Once Upon a Time in Hollywood
Best Actor -- Adam Sandler, Uncut Gems
Best Actress -- Renée Zellweger, Judy
Best Supporting Actor -- Brad Pitt, Once Upon a Time in Hollywood
Best Supporting Actress -- Kathy Bates, Richard Jewell
Best Original Screenplay -- Josh Safdie, Benny Safdie, Ronald Bronstein, Uncut Gems
Best Adapted Screenplay -- Steven Zaillian, The Irishman
Breakthrough Performance -- Paul Walter Hauser, Richard Jewell
Best Directorial Debut -- Melina Matsoukas, Queen & Slim
Best Animated Feature -- How to Train Your Dragon: The Hidden World
Best Foreign Language Film -- Parasite
Best Documentary -- Maiden
Best Ensemble -- Knives Out
Outstanding Achievement in Cinematography -- Roger Deakins, 1917
NBR Icon Award -- Martin Scorsese, Robert De Niro, Al Pacino, The Irishman
NBR Freedom of Expression -- For Sama
NBR Freedom of Expression -- Just Mercy