On Monday, the company’s chief executive Elon Musk tweeted that production had restarted and he would be “on the line with everyone else”.
US states and local governments are trying to determine the best way to open up after lockdown.
Mr Musk previously vowed to move the firm’s headquarters out of California if the plant was not allowed to reopen.
He has been vocal about the lockdown orders in recent weeks.
Mr Musk recently celebrated plans to relax restrictions across the country, writing on Twitter: “FREE AMERICA NOW”. He has also dismissed as “dumb” concerns about the coronavirus.
While the state has eased restrictions to allow manufacturing, Alameda County, where the Fremont plant is located, has not. The town is about one hour south of San Francisco.
On Saturday, Elon Musk said that Tesla had filed a lawsuit against the county asking a court to remove the order that prevents the carmaker from resuming production.
Rather than wait for a ruling, Mr Musk announced on Twitter on Monday that the plant would reopen.
The local police department said that it was aware of the situation, but that it would act at the discretion of county health officials.
The Alameda County Public Health Department said on Monday it was “actively communicating” with Tesla about reopening plans and that it was taking the same approach it had taken with other business that had violated lockdown orders.
In an email seen by Reuters, Tesla also reportedly told workers the decision to reopen was in line with California guidelines.
Mr Musk wrote on Twitter that Tesla had been “singled out”, saying that other US carmakers were allowed to restart production.
Other carmakers had planned to resume production in May but some have had to delay this in states like Michigan where non-essential business operations are limited.
Pictures of the Tesla car park on Monday showed it mostly full. The plant has been closed to all but limited essential operations since 26 March.