The person infected returned to L.A. County after traveling to South Africa via London on Nov. 22, the county health department said in a news release.
"The individual, who is a fully vaccinated adult and a Los Angeles County resident, is self-isolating, and their symptoms are improving without medical care," the news release said. "A small number of close contacts in Los Angeles have been identified and, to date, all have tested negative and have no symptoms."
Health officials added that the infection is most likely travel-related.
Earlier on Thursday before the case was confirmed, county Public Health Director Barbara Ferrer said that it would only be a matter of time before the omicron variant would be detected in the region.
"While we can't know for certain the impact of Omicron at this time, the good news is that we already know how to reduce transmission and slow spread using both pharmaceutical and non-pharmaceutical interventions," Ferrer said in a statement after the case was reported.
"I encourage everyone to take the steps that we know offer protection, including getting vaccinated or boosted, tested if you feel sick or are a close contact, and wearing your mask indoors and at large mega events," Ferrer added.
The patient in L.A. County is the second known case of the variant in California. The first U.S. case of the variant was reported in a San Francisco resident on Wednesday.