Clinton had a 13 percentage point lead over Sanders with more than half of the vote counted in California.
The size of her victory was a bit unexpected, as recent polls showed the race tightening. Clinton cut campaigning short in New Jersey last week to make a series of appearances in the Golden State.
Sanders had spent the past couple of weeks in California, hopeful that a victory in the Golden State would give him a greater rationale to continue on to the Democratic National Convention in July.
But Clinton’s campaign looked to claim not just a majority of delegates, but a majority of pledged delegates. Her campaign was bolstered by an overwhelming victory in New Jersey and a closer win in New Mexico. She also won South Dakota, while Sanders won races in North Dakota and Montana.
Earlier in the evening, Clinton claimed the nomination in a speech to supporters in Brooklyn, N.Y.
“Thanks to you, we have reached a milestone, the first time in our nation’s history that a woman will be our party’s nominee,” she said.
Sanders, speaking in Santa Monica, Calif., said that he would continue to press on and compete in the final primary next week, in Washington DC. He is scheduled to meet with President Obama on Thursday.
GOP presumptive nominee Donald Trump easily beat his Republican rivals to win the Golden State. His last rival, John Kasich, suspended his campaign last month.
In other races, Democrats California Attorney General Kamala Harris and Rep. Loretta Sanchez (D-Calif.) will face off in November in the race to succeed Sen. Barbara Boxer (D-Calif.). A Republican failed to make the general election ballot under California’s open primary system, in which the top two vote getters in the primary advance no matter the party.