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".
McGuane said she found relief knowing that the truth about her mother's death was finally known to the public.
In what may have been her final interview before her death, Kidder told WWJ's Sandra McNeill on May 9 she was "sad but not that sad" when her "Superman" co-star Christopher Reeve became a quadriplegic.
"It's a very unique sort of grief and pain", McGuane said.
She also appeared in the film The Great Waldo Pepper and the children's TV series RL Stine's The Haunting Hour.
Kidder, originally from Yellowknife, Canada, was a political activist who was arrested in 2011 in a Washington DC protest over the proposed Keystone XL pipeline from Canada's oil sands. The Canadian-born actress had always been open about drug and alcohol addictions, which she said partially triggered her very public breakdown in 1996.
The calls include responses by ambulances five times in seven months, including at the time of her death.
The actress died at the age of 69 at her home in Livingston, Montana, on 13 May. She experienced a very public manic episode in April 1996 when a computer virus caused her to lose three years' worth of drafts of her autobiography. I want what I know about her to be out there because it was glorious.
"The reality of my life has been grand and wonderful, punctuated by these odd blips and burps of madness", she said.
If you're anxious someone you know may be at risk of suicide, you should talk to them about it, says the Canadian Association for Suicide Prevention.