Mara, 61, and actress known for her roles on ER, Ray Donovan and Shameless, may have slipped, causing her to drown after her swim in the St. Lawrence River in Cape Vincent on Sunday.
She appeared in dozens of movies and television shows in a career of more than 30 years. The police said they believed she died in a drowning accident.
Mary Mara's cause of death was confirmed to be "an accident" due to "asphyxiation, and water drowning" while she was swimming in a New York river on Sunday.