The majority of the deaths are reported to have happened in Montecito, the wealthiest district in Santa Barbara.
Upmarket Carpenteria, a 30-minute drive from the city of Santa Barbara, has also been badly affected.
The mudslides were triggered by heavy rainfall with half an inch of rain falling in five minutes in Montecito, while Carpinteria received nearly an inch in 15 minutes.
The California coastline was subjected to a series of intense wildfires last month which left the area vulnerable to mudslides.
Where are the worst affected areas in Santa Barbara?
Montecito is said to be worst affected by the flooding disaster.
It is home to A-list celebrities including Oprah Winfrey, Ellen DeGeneres and Rob Lowe.
Measuring 23.99km sqm, it is bordered by major roads, the Pacific Coast shoreline and a coastal plain south of the Santa Ynez Mountains.
Around 7,000 residents were ordered to leave their homes by county officials with 23,000 evacuating voluntarily but not everyone managed to escape.
A 14-year-old girl was found alive amid the rubble of her Montecito home by firefighters searching with rescue dogs, the Los Angeles reported.
Around 300 people were left stranded in a canyon, while dozens of people were reported missing with the death toll expected to rise.
Santa Barbara County Sheriff Bill Brown said: ”The best way I can describe it is, it looked like a World War One battlefield.”
Normally fertile and green, with hot springs, shocking scenes now show the beautiful district covered in mud which has devoured residential blocks and Highway 101 – a major north-south route along the west coast of the USA.
The landslide has swept through residential districts covering luxury homes and Highway 101, a major north-south route along the coast.
Carpenteria is a small oceanside city located in southeastern Santa Barbara County, about a 30 minute drive away.
NWS (National Weather Service) Meteorologist Courtney Obergfell tweeted: “Not hard to see why #Montecito and areas below #ThomasFire are experiencing devastating mudslides.
“Over 5” of rain in a short time period. #Carpenteria Creek spiked up 20 feet #castorm”