Testing suggested the Rockland County case of the highly contagious virus may have originated outside of the United States, the department said in a statement.
"We are monitoring the situation closely and working with the New York State Department of Health and the Centers for Disease Control and Prevention to respond to this emergent public health issue to protect the health and wellbeing of county residents," Rockland County Health Commissioner Patricia Schnabel Ruppert said in a statement.
The CDC, which confirmed the case, has said no cases of polio have originated in the US since 1979. However, the virus has been brought into the country by travellers with polio. The last time this happened was in 1993, it said.
Polio symptoms include flu-like symptoms such as a sore throat, fever, tiredness and nausea, the CDC said.
Polio invades the nervous system and can cause irreversible paralysis within hours. It cannot be cured, but infection can be prevented by vaccination - and a dramatic reduction in cases worldwide in recent decades has been due to intense national and regional immunisation campaigns in babies and children.
In the late 1940s before polio vaccines were available, outbreaks of the virus disabled about 35,000 Americans each year, especially children and those who live in areas where sanitation is poor.
In June, polio was detected in sewage samples in London, the first sign since the 1980s that the virus could be spreading in England, but no cases have been found, authorities said.