"I am requesting that all things be done necessary to address this state of emergency declaration, effective immediately," Weaver told the City Council.
Exposure to lead can cause behavior problems and learning disabilities in young children. Genesee County earlier declared a public health emergency. Officials have told Flint residents not to drink unfiltered tapwater.
Flint switched from Detroit's water system last year to Flint River water in a cost-cutting move while under state emergency financial management. The Flint River was supposed to be an interim source until the city could join a new system getting water from Lake Huron.
But residents complained about the taste, smell and appearance of the water. Officials maintained that the water met safety standards, but children were found to have elevated levels of lead in their blood, and it was determined that corrosive river water was drawing lead from aging pipes.
Flint returned to Detroit's system in October.
Weaver was elected in November, unseating the incumbent, who led the city during the public health emergency and blamed state and federal agencies for the city's water problems. She promised while campaigning to issue an emergency declaration.
City Council members were divided about what the declaration will mean, The Flint Journal reported. Councilman Scott Kincaid said it's needed to seek more aid, noting that "we have to prove ... that we need resources."
Councilman Josh Freeman said he doesn't want residents to expect immediate help with the city's water infrastructure, including lead service lines, because of the declaration. He said the declaration doesn't fix the problem.
"We need to find a way to actually fix the problem," Freeman said.
The Associated Press