A joint statement from the coroner's office and her family urged "those suffering from mental illnesses, addiction and/or suicidal thoughts to seek appropriate counselling and treatment".
She continued to act in her later years but her popularity waned after she suffered a highly publicised mental breakdown in 1996. A cause of death was not originally reported, but Kidder's manager, Camilla Fluxman Pines, said at the time that she had passed away peacefully in her sleep. Additionally, ambulances were called to Kidder's home five times in seven months, including at the time of her death.
Kidder was found unresponsive at her home in Livingston on May 13. She believes it is important to be open and honest about suicide and not hide it.
Kidder was married to novelist Thomas McGuane with whom she had a daughter, Maggie.
A statement released on Wednesday by the Park county coroner Richard Wood said the star died from a self-inflicted drug and alcohol overdose, and no further details would be released.
"It's a very unique sort of grief and pain", she said. "Knowing how many families in this state go through this, I wish that I could reach out to each one of them".
Kidder delivered what many consider to be the quintessential portrayal of Lane, her gutsy, willful, confident attitude providing the ideal foil for Reeve's timid and mild-mannered Clark Kent. "The challenges that she had were very public". She wants to avoid any "cloud of shame". "She was really a blazing energy".
Kidder struggled with mental illness for a big chunk of her life, with events getting worse following a vehicle accident in 1990 that left her in debt and led her to using a wheelchair.
Most recently, Margot appeared in The Neighbourhood, which came out earlier this year.