Attorney General Xavier Becerra said in a statement that his office is "committed to defending California's common sense gun laws" and is reviewing the decision and evaluating its next steps.
The goal of the California law is to deter mass shootings, with Becerra previously listing as an example the terrorist assault that killed 14 and injured 22 in San Bernardino.
Benitez, an appointee of Republican President George W. Bush, called such shootings "exceedingly rare" while emphasizing the everyday robberies, rapes and murders he said might be countered with firearms.
The Giffords Law Center to Prevent Gun Violence, named after a former congresswoman who survived a mass shooting, is also still evaluating whether the decision applies more broadly, said staff attorney Ari Freilich.
But Freilich predicted the "extreme outlier decision" will be overturned on appeal and criticized a judge "so deeply out of touch that he believes mass shootings are a 'very small' problem in this country."
Becerra previously said similar Second Amendment challenges have been repeatedly rejected by other courts, with at least seven other states and 11 local governments already restricting the possession or sale of large-capacity magazines. The conflicting decisions may ultimately be sorted out by the U.S. Supreme Court.
Benitez ruled that magazines holding more than 10 rounds are "arms" under the U.S. Constitution, and that the California law "burdens the core of the Second Amendment by criminalizing the acquisition and possession of these magazines that are commonly held by law-abiding citizens for defense of self, home, and state."
Benitez described three home invasions, two of which ended with the female victims running out of bullets.
In the third case, the pajama-clad woman with a high-capacity magazine took on three armed intruders, firing at them while simultaneously calling for help on her phone.
"She had no place to carry an extra magazine and no way to reload because her left hand held the phone with which she was still trying to call 911," the judge wrote, saying she killed one attacker while two escaped.
The magazine ban was included in 2016 legislation that voters strengthened with their approval of Proposition 63, which was championed by then-Lt. Gov. Gavin Newsom.
In a statement, Newsom criticized the judge's ruling.
"This District Court Judge's failure to uphold a ban on high-capacity magazines is indefensible, dangerous for our communities and contradicts well-established case law," the governor said. "I strongly disagree with the court's assessment that 'the problem of mass shootings is very small.' Our commitment to public safety and defending common sense gun safety laws remains steadfast."