Kidder, who played Lois Lane – the love interest of Christopher Reeve’s Man of Steel, was found by a friend at her Montana home on 13 May.
At the time, her manager said Kidder, 69, had died peacefully in her sleep.
But a statement by Park County coroner Richard Wood said no further details would be released apart from the fact she died as a result of a “self-inflicted drug and alcohol overdose”.
“It’s a big relief that the truth is out there. It’s important to be open and honest so there’s not a cloud of shame in dealing with this,” said her daughter Maggie McGuane.
Kidder had suffered with mental illness for many years.
She suffered a breakdown in the 1990s and disappeared for four days in a highly publicised episode and was later diagnosed with bipolar disorder.
As well as four Superman movies, Kidder appeared in Brian De Palma’s Sisters and The Amityville Horror, and continued with small film and TV roles until last year.
She won a daytime Emmy award in 2015 for her role in children’s series RL Stine’s The Haunting Hour.
The Canadian actress was married three times and also dated the country’s former prime minister Pierre Trudeau, director Steven Spielberg and comedian Richard Pryor.
Her final years were marked by conflicts with down on their luck people she took into her home.
Police records obtained by Associated Press show authorities were called to her property 40 times between August 2016 and May 2018 over reports of theft, trespassing and other disturbances.
Five ambulances were also called in seven months.
Among those paying tribute when she died in May was Star Wars actor Mark Hamill.
He tweeted: “On-screen she was magic. Off-screen she was one of the kindest, sweetest, most caring woman I’ve ever known.
“I’ll miss you #MargoKidder. Your legacy will live on forever.”
:: Anyone feeling emotionally distressed or suicidal can call Samaritans for help on 116 123 or email email@example.com in the UK. In the US, call the Samaritans branch in your area or 1 (800) 273-TALK.