According to a report from the New York State Police, the actor was discovered in the St. Lawrence River in Cape Vincent on Sunday evening.
A preliminary investigation suggested the actor, who lived in the vicinity, had gone swimming and died by drowning.
In a statement, her manager described Mary as “one in a million”.
Mary Mara (Photo: J. Vespa via Getty Images)
“Mary was one in a million - one of the best actresses I ever met,” her manager, Craig Dorfman, said in a statement.
“I saw her for the first time off-Broadway in 1992 in Mad Forest and she was electrifying. She was unique, wickedly funny and a wonderful woman who will be missed.”
A cause of death has not yet been released, pending an investigation from the Jefferson County Medical Examiner’s Office. Authorities said that Mary’s body showed no signs of foul play.
Mary pictured at the 2006 Los Angeles Film Festival. (Photo: Matthew Simmons via Getty Images)
The actor, who had over 80 screen credits to her name, held a recurring role as Loretta Sweet on ER from 1995 to 1996. She also made appearances on The West Wing, Ally McBeal, Hope & Gloria, NYPD Blue and Dexter.
Outside of TV, her film credits include 1990′s Blue Steel and 1991′s True Colors.
She is survived by her stepdaughter Katie Mersola, sisters Martha Mara and Susan Dailey, brother-in-law Scott Dailey and nephew Christopher Dailey.
This article originally appeared on HuffPost UK and has been updated.