The statewide directive, which Governor Gavin Newsom said goes into effect "this evening", is the most dramatic containment measure yet by a U.S. state.
The wider USA, meanwhile, has 14,250 cases of the virus and 205 deaths, according to a running count being conducted by Johns Hopkins University in Baltimore, Maryland.
No end date was set for the measure, as the pandemic is a "dynamic situation", he said.
California has been among the worst hit by the virus, with at least 18 confirmed deaths so far, according to a Johns Hopkins University tally.
About 1,000 Californians have tested positive.
He did offer a glimmer by saying he did not expect it would last "many, many months". "I don't believe the people of California need to be told through law enforcement that it's appropriate just to home-isolate, protect themselves", Newsom stated. Residents will be allowed to leave home for essential needs including groceries and medical supplies, while workers in positions deemed essential are cleared to go to work.
Nirbhaya convicts to be hanged tomorrow, court dismisses plea seeking stay
However, none of the four convicts - Mukesh, Vinay, Pawan Gupta and Akshay Kumar Singh - recorded any will, the official said. Ram Singh, the sixth accused, allegedly committed suicide in the Tihar Jail days after the trial began in the case.
"We want people to go to essential jobs - we want people to deal with their essentials in a thoughtful and judicious way". It keeps open gas stations, pharmacies, grocery stores, restaurants doing take-out, banks, laundromats, and any essential government services.
Restaurants can still provide take-out and delivery, and food banks will still operate.
The Democratic governor said parts of the state were seeing infection rates double every four days.
The order came as models for the coronavirus outbreak estimated 56% of Californians - or over 22 million people - will become infected, with the state's hospital capacity at 20,000 fewer beds than will be needed at the peak of the epidemic.
The new stay-at-home order is meant to reduce that figure, Newsom said Thursday. California Highway Patrol has repeatedly said "no".
Of this amount, the state will spend $30 million to lease two medical facilities - Seton Medical Center in Daly City and St. Vincent Medical Center in Los Angeles, Newsom said in a statement. A prior order prohibited any indoor gathering of larger than 10 people.
Newsom on Thursday also wrote a letter to Congress requesting "more than $1 billion in initial funding" for California to tackle the pandemic.