Eristoff served as a member of the New York City Council from 1993 until 1999, elected three times to represent an overwhelmingly Democratic district on Manhattan's East Side. In addition to focusing on quality-of-life issues (he wrote the Taxi Rider's Bill of Rights and a law to ban panhandling near automated teller machines, for example), Eristoff became the Council's leading expert on technology and was made Chairman of the Council's Task Force on Technology in Government, the first Republican to hold a chair in modern times. In 1999, Eristoff became New York City Commissioner of Finance under Mayor Rudy Giuliani. He served in that role until 2002, earning recognition for implementing management strategies that leveraged technology and performance measurement to reduce costs, increase collections, and improve customer service.