The entertainment site quoted the Park County Coroner as saying that Kidder died from an intentional drug and alcohol overdose.
Kidder struggled with mental illness through a good part of her life, a situation that was made worse by a 1990 vehicle crash that left her in debt and led to her using a wheelchair for nearly two years.
The 69-year-old actress was found dead at her home in Montana on 13 May.
The AP also reports that the Superman actress was living in a state of chaos in her final years, as her attempts to provide refuge for those in need resulted in authorities being called to her home 40 times between August 2016 and her death. She believes it is important to be open and honest about suicide and not hide it.
McGuane noted that Montana has one of the highest suicide rates in the nation and she urged people with mental illness to seek help.
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"It's a very unique sort of grief and pain".
Born Margaret Ruth Kidder on October 17, 1948 in Canada's Northwest Territories, she grew up in a small town without a movie theatre.
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.
She rose to fame as Lois Lane in 1978's Superman, and followed that massive success with Superman II in 1980, Superman III in 1983 and Superman IV: The Quest for Peace in 1987.
McGuane said she found relief knowing that the truth about her mother's death was finally known to the public.
Kidder, like Carrie Fisher, was an advocate for ending the social shame surrounding mental health issues.