The new figure broke an old record set just the day before of more than 37,000 new COVID-19 cases.
The county also reported 28 additional deaths from the virus.
LA County continues to face an increased strain on the local healthcare system driven by a shortage of hospital workers as many of them contract the virus and have to stay away from the workplace.
The continued danger posed by the virus to young people was demonstrated in Orange County, which is now reporting its third death in a child under 5 years old from COVID-19.
"COVID does affect the children and unfortunately this child was too young to get vaccinated," said Dr. Regina Chinsio-Kwong with the Orange County Health Care Agency.
COVID-related hospitalizations of children under 5 are spiking across the country - now hitting their highest level ever.
Experts say it underscores the need for eligible people - older kids and adults - to get their shots.
"This is why it is so important for everyone who is eligible to get vaccinated to hopefully reduce or prevent another young, ineligible person from getting COVID and dying from COVID," said Chinsio-Kwong.
Results of a massive CDC COVID study are showing the importance of vaccinations.
That study looked at more than 1 million vaccinated adults before the omicron variant appeared and found that only a tiny fraction of a percent of vaccinated adults suffered severe COVID-related illnesses.
And an even smaller percent died.
Those who did become severely ill or die were primarily older adults, immune suppressed, or people with underlying medical conditions.