The Orange County Democratic Party said it's the first time since 1940 that all seven House seats in the county, home to 3.2 million people, are in Democratic control.
Three seats all or partly in the northwestern end of the county are held by Democrats who were easily re-elected.
Democrats also recently picked up the last Republican-held House seat anchored in Los Angeles County, when Democrat Katie Hill ousted Republican Congressman Steve Knight.
Republicans also lost a seat in the agricultural Central Valley.
With other gains, Democrats will hold a 45-8 edge in California US House seats next year.
The 39th District was one of seven targeted by Democrats across California after Hillary Clinton carried them in the 2016 presidential election.
Cisneros, 47, a US$266 million lottery jackpot winner, had been locked in a close race with Kim in a district that has grown increasingly diverse. It's about equally divided between Republicans, Democrats and independents, as it is with Asians, Hispanics and whites.
"In one of the most diverse districts in the country I learned that for all of our differences, we all care about the same things," said Cisneros, who will be the first Hispanic to represent the district.
"Most of all, we want to live in a world brought together by hope, not divided by hate," he said in a statement.
Kim, 55, a former state legislator, worked for years for retiring Republican Congressman Ed Royce, who is vacating the seat and had endorsed her.
In a state where US President Donald Trump is unpopular, Kim sought to create distance with the White House on trade and healthcare. Her immigrant background — and gender — made her stand out in a political party whose leaders in Washington are mostly older white men.
"I'm a different kind of candidate," she had said.
It wasn't enough. Democratic ads depicted her as a Trump underling, eager to carry out his agenda.
Cisneros, a first-time candidate, described his interest in Congress as an extension of his time in the military, saying it was about public service. He runs a charitable foundation with his wife.
On healthcare, he talked about his mother who went without insurance for 16 years. "That should just not happen in this country," he had said.
While the election delivered mixed results around the US, it affirmed California's reputation as a Democratic fortress.
Democrats are on track to hold every statewide office — again. The party holds a supermajority in both chambers of the Legislature and has a 3.7-million advantage in voter registration.
There wasn't even a Republican on the ballot for US Senate.
In Florida, the news isn't so good for Democrats.
Democratic Senator Bill Nelson has ended his bitterly close re-election bid by conceding the race to Republican Rick Scott.
Official results showed that the three-term incumbent trailed Scott by more than 10,000 votes.
Yesterday Democratic candidate for governor Andrew Gillum conceded to Republican Ron DeSantis.
- AP
