With 98 percent of the vote counted, unofficial results show the Democrat Rice defeating Republican Howard Sturim, 59 percent to 41 percent.
On the campaign trail, Rice trumpeted her enforcement of drunken driving laws in the suburban county east of New York City.
Sturim worked in the county prosecutor's office for Rice's predecessor, Denis Dillon.
In neighboring Suffolk County, District Attorney Thomas Spota was re-elected to a fourth, four-year term without opposition. He had the endorsement of both major parties.
A court ruling earlier this year struck down a term limits law in the county, saying it did not apply to the district attorney.