The county said Brandau's test came back positive on Thursday, and he will quarantine for ten days.
The Fresno County Board of Supervisors Office, Clerk of the Board and Administrative offices will temporarily close while staff member quarantine.
About 35 people have been quarantined at the Fresno County Hall of Records.
On Tuesday, Brandau said he started having a scratchy throat.
He asked an assistant for a cough drop and from there his symptoms progressively got worse.
Speaking from his home, Brandau said he's spent the last 24 hours remembering where he's been and who he needs to call to notify.
On election night, Brandau attended a gathering with close to a dozen others at a private residence.
"Unfortunately, now I know I had COVID-19 at that time and so I exposed all of my colleagues in the board meeting, the other supervisors, the county clerk, the CAO, the county attorney, and others. And their staffs," he told Action News.
Among the guests in attendance were Mayor Lee Brand, Councilmember Mike Karbassi, and Mayor-Elect Jerry Dyer. All are now taking various precautions.
"Most of us were notified yesterday by Steve that he had tested positive, which is why I got tested last night just to be sure. I didn't have any symptoms, no fever, no coughing or anything of that nature but knew that I had been exposed and wanted to have the most reliable test," said Dyer.
Dyer said he took the molecular test that has been shown to have a more reliable accuracy rate. The results were negative.
Contact tracers have already spoken with Brandau and gone over each place he visited recently. It included trips to various stores like Walmart and Lowe's. He doesn't know where he picked it up. But he said it's unfortunate the widespread impact it's having on his friends and colleagues.
Other city officials who either had contact with Brandau - or were with others who spent time with him are also now working from home - out of an abundance of caution.
The Board of Supervisors have all taken tests and are awaiting the results.
They will decide next week whether to move forward with their November 17th board meeting or postpone it.
The Fresno County Administrative Officer Jean Rousseau released the following statement:
"Upon learning of the positive test, we took quick and appropriate action per our safety protocols to close our offices and send staff home to protect against any potential spread of the virus. We believe the risk to be minimal thanks to health and temperature screenings and mask requirements that are in place, however our top priority is always to protect the health and safety of our staff and those whom we serve and these actions are the right thing to do."
Fresno City Councilmember Miguel Arias said City Hall officials would also be quarantining after possibly being "exposed to a person, not a City official" who had tested positive for COVID-19.
No further information was provided.