Gavin Newsom was among a group of more than 12 people who celebrated the birthday of his longterm political advisor Jason Kinney, according to the San Francisco Chronicle.
The group at the 6 November party reportedly included people from several different households, with California currently limiting events to no more than three households.
A spokesperson for Mr Newsom told the Chronicle that the dinner did not violate state restrictions as it was held outdoors at the Napa venue.
And they insisted that everyone followed guidelines and wore face masks.
State guidance does not specify if the three household limitation applies to outdoor dining, according to Fox2.
But the state’s Department of Health has mandated that multiple household gatherings are held outside, where transmission risks are lower, and last for less than two hours.
With coronavirus cases surging in California state officials have urged residents to not travel for Thanksgiving, and said they should quarantine for 14 days if they do.
California became the second state, after Texas, to pass one million Covid cases.
“California just surpassed a sobering threshold — 1 million Covid-19 cases — with no signs of the virus slowing down,” said Mr Newsom in a statement.
“Increased cases are adding pressure on our hospital systems and threatening the lives of seniors, essential workers and vulnerable Californians.
“Travel increases the risk of spreading Covid-19, and we must all collectively increase our efforts at this time to keep the virus at bay and save lives.”
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