His narrow victory over Democrat Thomas Suozzi, and a Republican gain of two county legislative seats, puts Republicans firmly back in control of Nassau.
Aides to Mangano, 47, said his speech will talk about the county's fiscal problems, a regional approach to economic development, and the need to provide services to military veterans.
"The speech is not doom and gloom. It's got some silver lining," said Michael Martino, who will be press secretary to Mangano. "But he's going to make clear there are some clouds on the horizon."