With 98 percent of the vote counted, unofficial results showed Mangano leading Suozzi 59 percent to 41 percent.
Mangano, 51, touted his tax-cutting policies as well as his handling of the aftermath of Superstorm Sandy, which caused tremendous destruction on Long Island's southern coast. He also put together a proposal to renovate the Nassau Veterans Memorial Coliseum, the NHL arena being vacated in 2015 by the New York Islanders.
"What a great night!" Mangano told supporters at his victory party. He said he felt "blessed" to continue in the job.
Suozzi, who lost a re-election bid to Mangano in 2009 by just a few hundred votes, said his failed comeback was "a tough loss for Democrats - a tough loss for me personally."
"So many people were so loyal, under such tough circumstances," he said.
The wealthy county just east of New York City has been under a state-imposed financial watchdog since 1999. In 2011 it declared a fiscal emergency and imposed a wage freeze on county workers that remains in effect.
Lawrence Levy, executive dean of the Hofstra University National Center for Suburban Studies, said this year's election was being watched beyond Long Island as a barometer for the 2014 midterm elections in Washington because it is viewed as a typical suburban swing county.