More than 50 people have been rescued from rooftops after trees and power lines blocked roads.
Among them was a mud-caked 14-year-old girl who was pulled after hours trapped under the rubble of a collapsed home in Montecito.
A family of five was also rescued with their two dogs from the rooftop of their house in Carpinteria.
Dozens of homes were destroyed, cars demolished and trees toppled after heavy downpours in communities along California's scenic coastline send mud, boulders and tons of debris rolling down hills.
Mandatory evacuations had been ordered in parts of Santa Barbara County, but it is believed only 10% to 15% of residents actually heeded the warning.
Montecito, one of the areas hardest hit by the mudslides, is a wealthy enclave of about 9,000 people - and home to celebrities including Oprah Winfrey, Rob Lowe and Ellen DeGeneres.
Winfrey said she was "praying for our community again in Santa Barbara" as she posted a video on Instagram showing the depth of mud outside her home.
Actor Rob Lowe tweeted: "Mourning the dead in our little town tonight. Praying for the survivors and preparing for whatever may come."
He had earlier tweeted that Winfrey's home was a "staging ground for helicopter rescues".
A massive operation is also under way to rescue 300 people who are stranded in Montecito's Romero Canyon area, and helicopters borrowed from the US Coast Guard are being used to airlift them out.
The first confirmed death was that of Roy Rohter, the founder of St Augustine Academy in Ventura. The Catholic school's headmaster Michael van Hecke said Mr Rohter's wife had been injured in the mudslide.
Bill Brown, sheriff of Santa Barbara, said: "The best way I can describe it is, it looked like a World War One battlefield.
"It was literally a carpet of mud and debris everywhere with huge boulders, rocks, downed trees, power lines, wrecked cars, lots of obstacles and challenges for rescue personnel to get to homes, let alone to get people out of them."
Thomas Tighe said he stepped outside his Montecito home in the middle of the night after hearing the "deep rumblling" sound of boulders moving as mud levels rose.
Two cars were missing from his driveway and he watched two other vehicles slowly move sideways down the middle of the street "in a river of mud", he said.
The flash flooding began in the Santa Ynez Mountains, which was badly hit by wildfires last month.
After a wildfire, burned vegetation and charred soil are especially susceptible to destructive mudslides because scorched earth does not absorb water well and the land is easily eroded when there are no shrubs.
The death toll has surpassed the number of fatalities from a California mudslide in January 2005, when 10 people were killed in the town of La Conchita.