Also joining the cast: Connie Britton, Jennifer Coolidge, Laverne Cox, Max Greenfield, Christopher Mintz-Plasse, Sam Richardson ("Veep"), and Molly Shannon.
Mulligan will play a young woman who takes revenge for her tragic past on all the predatory man who crosses her path.
The movie has already started production in Los Angeles. It will mark the feature directorial debut of Emerald Fennell, who will also write and produce the film.
Fennell wrote three episodes of "Killing Eve." Her acting credits include "Call the Midwife" and young Camilla Parker Bowles on the upcoming season of Netflix's "The Crown."
Her short film, "Careful How You Go," was nominated for the Grand Jury Prize at the 2018 Sundance Film Fest.
"I am beyond excited to be working with so many impossibly talented and wonderful people on 'Promising Young Woman,'" Fennell said. "Frankly, I feel lucky to be allowed anywhere in the vicinity of so many geniuses, and I can't wait to bring this twisted tale to life."