Pacific Gas & Electric (PG&E) admitted the 2018 Camp Fire, the state’s deadliest and most destructive, was caused by its faulty equipment.
In the court hearing, a judge read the name of each victim aloud to the company chief executive.
The company will be fined millions of dollars, but no-one will go to jail.
Many of the Camp Fire’s victims were elderly or disabled.
A number of them were found in burnt-out cars, killed as they attempted to flee the blaze with their family and neighbours.
Others were discovered in and around their homes, as some elderly residents decided against leaving early, not understanding the gravity of the threat.
In Butte County Superior Court on Tuesday, an image of each victim was displayed on a screen as PG&E’s chief executive Bill Johnson pleaded to every single count of involuntary manslaughter, responding 84 times: “Guilty, your honour.”
In a highly unusual US corporate acknowledgment of criminal wrongdoing, Mr Johnson apologised to the families, saying: “I’ve heard the pain and anguish.
“No words from me can ever reduce the magnitude of that devastation.
“We know we cannot replace all that the fire destroyed.”
But some survivors have condemned the plea deal as a slap on the wrist for PG&E, which has been linked to several dangerous blazes since 2015.
They accuse state officials of long failing to hold the utility accountable because of its political clout.
The largest publicly held utility in the US, it has to pay a maximum fine of $3.5m (£2.7m) and cover the $500,000 costs of the investigation.
PG&E also admitted one count of unlawfully starting a fire because it did not properly maintain the power line, which ignited in a forested area known for strong winds.
Eighty-five people lost their lives due to the Camp Fire, though one death was ruled a suicide.
The blaze devastated the town of Paradise, located north-east of San Francisco, which some say will take up to a decade to fully rebuild.
Over 10,000 homes were burned down and more than 153,000 acres razed.
PG&E will be sentenced later this week.
The utility has already committed to settle claims from insurers and local government agencies for more than $25bn.
That includes a $13.5bn settlement with fire victims.
The agreements are expected to pave the firm’s exit from its 2019 bankruptcy filing.
In 2016, PG&E was convicted of safety violations and obstruction of an investigation into a 2010 gas pipeline explosion that killed eight people in 2010 in a town south of San Francisco.
In 1996, the firm had to pay one of the largest sums ever awarded in a US class action lawsuit after activist Erin Brockovich sued over the chromium poisoning of residents in Hinckley, California.
The Camp Fire began in November after a suspension hook on a nearly century-old tower broke and caused a power line to fall and release sparks on to dry grass in the Sierra foothills.
PG&E says it has since made changes to ensure a safer power grid, including replacing old poles and clearing flammable vegetation.
But it will also continue the unpopular power cuts during dangerous weather conditions that it began last year.