The coroner in Park County, Mont. said on Wednesday that Kidder "died as a result of a self-inflicted drug and alcohol overdose".
As fans of the actress know, she became a huge star in the 1970's for her role as Lois Lane, the girlfriend of Clark Kent - Superman.
The coroner did not specify what drugs she took.
Kidder's daughter, Maggie McGuane, said she knew that her mother had taken her own life.
Kidder was diagnosed with bipolar disorder and eventually suffered a public nervous breakdown in 1996 after facing several career setbacks in the early 1990s, including a vehicle crash which resulted in her being paralyzed for two years.
The 69-year-old actress was found dead at her home in Montana on 13 May.
Kidder was married to novelist Thomas McGuane with whom she had a daughter, Maggie.
At the time, Kidder's manager, Camilla Fluxman Pines, said Kidder died peacefully in her sleep. She wants to avoid any "cloud of shame".
McGuane went on to tell the AP that Montana has "one of the highest suicide rates in the nation" and urges those with mental illness to seek help.
The actress had publicly struggled with mental-health issues. She continued to act in many movies and TV shows, most recently the 2017 film "The Neighborhood".
She says it's a unique sort of grief and pain and that she would like to reach out to every family who is suffering through a loss by suicide. Additionally, ambulances were called to Kidder's home five times in seven months, including at the time of her death.
Both Kidder and co-star Christopher Reeve, who played Superman, were relative unknowns when they got their leading parts, and neither saw many major roles afterwards.
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