George Maragos leads Howard Weitzman, who preceded him in office. Maragos also defeated Weitzman two years ago.
The two were neck and neck heading into Election Day, according to polls. But Maragos was ahead 53 percent to 47 percent, according to unofficial returns.
"This is a great victory for the residents of Nassau County," Maragos said. "They spoke loud and clear, and we will hold the line on taxes for the next four years to protect them and improve our economy."
The 64-year-old previously Maragos headed a Manhattan financial technology firm.