No information was released about the teen's symptoms or travel history.
In a news release, County Executive George Latimer said: "We are concerned about this discovery, and are closely monitoring the situation. With the new variant circulating, the best way we can protect our families and ourselves is by being vaccinated, and by getting a booster shot if you are eligible."
Latimer has declared a state of emergency for Westchester County due to the omicron and delta variants.
He warned: "I know you are tired but we - together - must take action to be better prepared to fight COVID and the new variants."
The state of emergency does not include any new mandates but Latimer warns that could change if conditions deteriorate.