Kevin de León (D), sets a 100 percent clean electricity goal for the state by 2045.
The new law, along with an executive order Brown signed directing California to take as much carbon dioxide out of the air as it emits, represent the latest in a string of ambitious environmental initiatives as California seeks to fill a void left by President Donald Trump's decision to withdraw from the Paris climate accord and his efforts to boost the coal industry.
Brown said in a statement as he signed the law in Sacramento, "California is committed to doing whatever is necessary to meet the existential threat of climate change". "It will not be easy".
He says California is doing the opposite of the federal government when it comes to climate policy.
Brown timed the signing of the bill to kick off this week's global climate summit that will bring officials and activists from around the world to San Francisco. It sets a new policy to make 100 percent of California's energy resources come from zero-carbon sources by 2045.
Brown is pushing for regional management of the western grid to better share renewables, but he was unable to convince lawmakers to give up the control that the governor and Legislature now have to appoint leaders of the state's primary grid operator.
At least 20 countries and twice as many large cities have made similar pledges, but California - the fifth largest economy in the world - is by far the biggest jurisdiction to do so to date.
California already gets a substantial portion of its electricity from renewable resources.
Opponents also noted that transportation - mostly gasoline and diesel fuel burned by cars and trucks - generates 41 percent of the state's greenhouse gas emissions, more than double the 16 percent that power plants produce.
As Mexico, Canada, and other USA states seek to lower emissions, North America will continue to be one of the largest energy storage markets in the world for many years.
"We all have the opportunity and the obligation to do our part to combat climate change", he told AFP hours after signing the bill into law. Before Monday, the target was 50 percent by 2030. Jerry Brown for setting a goal of phasing out fossil fuels from the state's electricity grid by 2045.
California has consistently beaten its clean-energy targets, and now consumes just 8 percent of the U.S.' electricity despite accounting for 12 percent of the country's population.
"We've reached all these great goals with renewables, but at the same time our families have paid the price with an increase in their electric bills every year", said Devon Mathis, R-Visalia, during the Assembly debate.
Under the terms of the legislation, all utility companies must get 60% of their energy from renewable sources by 2030.