According to the New York State Police, the actress — known for "ER" and "Ray Donovan" — drowned during a swim in the St. Lawrence River on Sunday.
The police were called to the scene for a possible drowning and found her body.
In a statement, the New York State Police said, "The preliminary investigation suggests the victim drowned while swimming. The victim's body showed no signs of foul play and was transported to the Jefferson County Medical Examiner's Office pending an autopsy to determine an official cause of death."
In another statement, her brother-in-law Scott Dailey revealed that Mara was staying at her sister Martha's summer home when she died.
Following her death, her manager Craig Dorfman remembered her as "one of the finest actresses I ever met."
He told People magazine, "I still remember seeing her onstage in 1992 in 'Mad Forest' off-Broadway. She was electric, funny, and a true individual. Everyone loved her. She will be missed."
Mara had recurring roles on "ER" (1995-1996), "Nash Bridges" (1996-1997), and "Ray Donovan" (2013).