Garcia, who lost his mother and stepfather to COVID, said he tested positive Monday morning "for the first time since the pandemic started."
"I have light symptoms and otherwise feel fine," the 44-year-old Garcia wrote on his Twitter account. "I plan to rest and work from home this week. Grateful to be vaccinated and boosted."
In July 2020, Garcia's mother, Gabriella O'Donnell, died due to COVID- 19. His stepfather, Greg O'Donnell, died two weeks later.
Garcia was elected Long Beach mayor in 2014, becoming the city's youngest and first openly gay mayor.
He will square off in a November runoff against Republican John Brisco in the race to represent the reconfigured 42nd Congressional District, which includes Long Beach and stretches north to areas including Lakewood, Bellflower, Downey and Huntington Park.