Deadline reported Thursday that Mara, 36, will play Hepburn in the project for Apple TV+.
The film is written by Michael Mitnick and directed by Luca Guadagnino (Call Me by Your Name). Mara will also serve as a producer.
The Hollywood Reporter confirmed Mara's casting and said the project is one of many new original films in the works at Apple TV+.
Hepburn was born in Belgium in 1929 and died at age 63 in 1993. She is known for such films as Breakfast at Tiffany's, My Fair Lady, Funny Face, Sabrina and Roman Holiday.
During her career, Hepburn was nominated for an Oscar for Best Actress five times, winning in 1953 for Roman Holiday.
Mara is known for starring in David Fincher's adaptation of The Girl with the Dragon Tattoo. She most recently starred in Guillermo del Toro's movie Nightmare Alley, which opened in theaters in December.