Mayor Eric Garcetti is now isolating in his hotel.
First Minister Nicola Sturgeon and her Welsh counterpart Mark Drakeford hosted the event along with Northern Ireland’s First Minister Paul Givan and Deputy First Minister Michelle O’Neill.
Boris Johnson made a brief appearance at the event on Tuesday in the blue zone at COP 26 in Glasgow.
A message on the official mayor of Los Angeles Twitter account posted on Wednesday afternoon said: “Mayor Garcetti tested positive for COVID-19 earlier today.
“He is feeling good and isolating in his hotel room. He is fully vaccinated.”
Among those in attendance at the breakfast were the prime ministers of Norway, Sweden, Barbados and Vietnam, as well as UK Foreign Secretary Liz Truss.
The presidents of Armenia, Costa Rica and Zambia also took part, as well as Mayor of London Sadiq Khan and Glasgow City Council leader Susan Aitken.
Everyone attending the COP 26 blue zone must take a lateral flow test each morning and increased hygiene procedures are in place at the venue.
Susan Aitken has booked a PCR test and is self isolating, however, FIrst Minister Nicola Sturgeon met him but was not a close contact.
A Glasgow City Council spokesman said: “Cllr Aitken was delighted to welcome Mayor Garcetti to Glasgow on Monday and wishes him a speedy recovery.
“She has not been identified as a close contact but, out of an abundance of caution, has booked a PCR test and will isolate at home until she receives a result.”