The doctor, Craig Spencer, was placed in isolation at Manhattan's Bellevue Hospital Center earlier Thursday after suffering Ebola symptoms including a 103-degree Fahrenheit (39.4 Celsius) fever and nausea.
New York Mayor Bill de Blasio confirmed Spencer's positive test at a late-night press conference, but urged residents not to panic, insisting the city was fully prepared to stop the disease in its tracks.
"We want to state at the outset, (this) is no reason for New Yorkers to be alarmed. Ebola is an extremely hard disease to contract. It is transmitted only through contact with an infected person's blood or other bodily fluids," said de Blasio.
New York Governor Andrew Cuomo said city officials had already identified four people believed to have crossed paths with Spencer, and said he did not expect a repeat of the situation in Dallas, where hospital staff appeared to have been caught off-guard by the arrival of the disease that has killed almost 4,900 people in West Africa.
"Dallas was caught before they could really prepare. We had the advantage of learning from their experience," he said.