In a statement, Brown said he acted based on the requests of Porter Ranch residents and the "prolonged and continuing" nature of the gas blowout.
The well, at SoCalGas' nearby Aliso Canyon underground storage facility, has been spewing up to 1,200 tons of methane daily, along with other gases. The leak was first reported in October.
SoCalGas is paying to relocate thousands of households after residents complained that the gases were making them sick. They say they have suffered nosebleeds, nausea, headaches and other ailments.
Los Angeles County and the Los Angeles school board already have declared the crisis a state of emergency, moving students out of two schools in the neighborhood.
Los Angeles City Attorney Mike Feuer filed a lawsuit against SoCalGas in December, claiming the ongoing leak is worsening climate change by emitting the same amount of greenhouse gas in one hour as 200,000 cars running for one year.
Resident Matt Pakucko, the head of a community group that has been pushing for the state declaration, said he cried when he heard about the governor's order.
Pakucko, speaking from a hotel where the gas company had moved him and his girlfriend, said he faced some doubts from his community about pressing the governor to declare an emergency. He said he had his own concerns that Brown might not take strong enough action, but was pleased in the end.
"I'm not worried about being validated or vindicated, but that people get what they need," he said. "And that's what we've been pushing for."
The governor's move, among other measures, directs efforts by California agencies in stopping the leak, and it orders emergency regulations for other gas storage facilities throughout the state.
The Aliso Canyon site is one of the country's largest underground natural gas storage facilities and can hold enough natural gas to fuel Southern California for a month.
Brown toured the site of the leak for the first time earlier this week. The governor previously has resisted local calls for a state declaration, saying he wanted to make sure the utility, rather than the state, bore the financial cost.