Attorney Kerry Lawrence said her body was found in New York on Robert F. Kennedy Jr.'s property in Mount Kisco, about 36 miles north of New York City. An autopsy was scheduled for Thursday, but sources told ABC News that she hung herself inside the family house.
"We deeply regret the death of our beloved sister Mary, whose radiant and creative spirit will be sorely missed by those who loved her," the family said in a statement. "Our heart goes out to her children who she loved without reservation."
The couple was married for 16 years. She was Robert F. Kennedy Jr.'s second wife.
Kennedy Jr., a prominent enviromental lawyer, is the son of the late Sen. Robert Kennedy and the nephew of President John F. Kennedy. The couple has four children together.
The family described Mary Richardson Kennedy as "a genius at friendship" in a statement issued by Robert Kennedy Jr.'s chief of staff.
"Mary inspired our family with her kindness, her love, her gentle soul and generous spirit," the statement said.
Mary Richardson Kennedy had trouble with drugs and alcohol, and had two high-profile arrests around the time her husband filed for divorce in 2010. She pleaded guilty to drunken driving later that year when police reported her seeing her car hit a curb outside a school near her home.
ABC News contributed to this report.
NOTE: It was reported that Mary Richardson Kennedy was Robert F. Kennedy, Jr.'s ex-wife, but the divorce papers that were filed in 2010 were never finalized.