Republican Mayor Larry Curry, an avid Trump supporter, had just declared last week that he wouldn't require masks.
Florida doctors penned a letter to Jacksonville Mayor Lenny Curry and city officials warning them that bringing the Republican National Convention to the city could cause coronavirus to spiral out of control unless the event is "very significantly reduced in numbers" or pushed back, First Coast News reports.
The new mandate, which goes into effect at 5 p.m. ET on Monday, raises the possibility that attendees could be required to wear face masks at the GOP convention.
Recently a group of almost 500 Florida-based doctors - many from northeast Florida - signed an open letter to Curry calling for the convention to be postponed.
"It is extremely unsafe and contrary to current public health recommendations to stage a large event in an area where the number of cases is surging".
Curry was reportedly absent from the news conference announcing the mandatory mask measure on Monday, with a spokeswoman telling the New York Times Miami Bureau Chief that he had a "family commitment".
President Donald Trump believes the decision to wear a mask to help prevent spreading the deadly coronavirus, now infecting record numbers of people in many places in the United States, is personal, White House Press Secretary Kayleigh McEnany said on Monday.
"Healthcare experts advise masks and cloth face coverings help slow the spread from individuals with COVID-19 to vulnerable populations including from those who are asymptomatic", city spokesperson Nikki Kimbleton said Monday.
Florida Gov. Ron DeSantis was asked Monday about Jacksonville's order.
He encourages people to make whatever decision is best for their safety.