The coroner did not specify which drugs she took, TMZ reported.
Kidder's daughter, Maggie McGuane, told the AP "it's a big relief that the truth is out there".
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. Her obituary at the time said simply that she passed away at her home, and does not mention her cause of death. Kidder later said she suffered a manic episode.
In a statement, the Park County, Montana, coroner's office confirmed to the wire service that the actress "died as a result of a self-inflicted drug and alcohol overdose". She believes it is important to be open and honest about suicide and not hide it.
McGuane urged people with mental illness to seek help.
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.
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.
"In her last months, she was herself - same kind of love, same kind of energy", Kesich said.
"The reality of my life has been grand and wonderful, punctuated by these odd blips and burps of madness", she said.
Kidder, like Carrie Fisher, was an advocate for ending the social shame surrounding mental health issues.
Most recently, Margot appeared in The Neighbourhood, which came out earlier this year.