Sarkozy spoke for about 50 minutes at the invitation of Brazil's BTG Pactual bank in Manhattan's luxury Waldorf Hotel, where tight security kept the event well out of the public eye.
Sarkozy has stayed out of the limelight since losing the presidency to the Socialist Francois Hollande in May, spending a lot of his time on holiday in Canada, Morocco and on the French Riviera with his wife, ex-supermodel Carla Bruni.
He has made just one public statement, criticizing Hollande's policy on the conflict in Syria.
Participants said Sarkozy began his remarks in English before switching to French, and he answered half a dozen questions.
"He was pretty relaxed, tanned. He talked about politics in Europe, of European history," said one participant, Jean-Michel Raynaud.
"He put the European crisis in the historical context that investors forget and also talked about emerging economies," another said. A third participant said Sarkozy was "very charismatic."
Sarkozy, wearing a blue suit and with a light beard, declined to take questions from waiting journalists as he arrived at the event. Earlier in the day he'd gone jogging in Central Park, out of sight of cameras.
He was due to leave New York on Saturday.
"He's very honored to have been invited to this conference," an aide said.
Former statesmen can earn large speaking fees, but it was not immediately known how much Sarkozy might have been paid for the New York appearance.
Back in France, opinions were split on whether the US trip was the start of a post-presidential career on the lecture circuit or if the French right-winger had secret plans to make a political comeback.