Last year, 82 people brought the virus into the US from other countries, representing the largest import of measles cases since 2000.
Mr Day also revealed some parents had been "resistant" towards the banning.
Day said he was taking the action in hopes of reversing a recent uptick in cases amid disturbing reports that health workers were encountering resistance when investigating cases. The ban will apply to places like restaurants, movie theatres and places of worships, but not outdoor public spaces like parks.
The ban in Rockland County, which starts at midnight on Tuesday (local time), will prohibit anyone under the age of 18 from public places within the county, said Rockland County Executive Ed Day, who called the outbreak the worst in the US. "We must do everything in our power to end this outbreak and protect the health of those who can not be vaccinated for medical reasons and that of children too young to be vaccinated".
Day noted that this is the first effort of its kind nationally.
"No one from law enforcement will be asking for vaccination records", Day explained. Parents will be held accountable if they are found to be in violation of this State of Emergency, and the focus of this effort is on the parents of these children. "We will not sit idly by while children in our community are at risk".
There have been 181 confirmed cases of measles in Brooklyn and Queens since October, a lot of them involving members of the Orthodox Jewish community, according to the CDC.
The cases are now clustered in eastern Ramapo, though officials warn exposure may occur anywhere in the county.
One-year-old Bella Huang cries as medical assistants secure her legs for a series of vaccines at the International Community Health Services clinic in Seattle, Washington, US, March 20, 2019.
This week, US District Court Judge Vincent Briccetti denied their request, ruling it wasn't in "public interest" to allow the children to go back to school. Health officials say the best way to stop the disease's spread is a vaccination rate in the community of 92 to 95 percent. This will help the facility take steps to ensure others won't be exposed to the virus.
"This is the greatest number of imported cases since measles was eliminated from the United States in 2000".