It was considered the deadliest single wildland blaze in state history, authorities said on Monday.
The latest death toll, up from 29 tallied over the weekend, was announced by Butte County Sheriff Kory Honea at an evening news conference in the nearby city of Chico after authorities located the remains of 13 additional victims from a blaze dubbed the Camp Fire.
That fire already ranked as the most destructive on record in California, having leveled more than 7,100 homes and other buildings since it erupted on Thursday in the Sierra foothills of Butte County, about 175 miles (280 km) north of San Francisco.
Honea said the number of people listed as missing in the disaster remained officially at 228, but added that his office had received more than 1,500 requests for "welfare checks" from people concerned about the fate of their loved ones. He said his office had managed to confirm the safety of the individuals in question in 231 of those cases so far.
More than 15,000 more structures remained listed as threatened on Monday in an area so thick with smoke that visibility was reduced in some places to less than half a mile.
The bulk of the destruction and loss of life occurred in and around the town of Paradise, where flames reduced most of the buildings to ash and charred rubble on Thursday night, just hours after the blaze erupted.
The 42 confirmed fatalities marks the largest loss of life ever from a single wildland fire in California, Honea said. It also far surpasses the all-time record number of deaths from a California wildfire - 29 in 1933 from the Griffith Park blaze in Los Angeles.
Authorities reported two more people perished over the weekend in a separate blaze, dubbed the Woolsey Fire, that has destroyed 370 structures and displaced some 200,000 people in the mountains and foothills near Southern California's Malibu coast, west of Los Angeles.
President Donald Trump on Monday approved a major disaster declaration for California at the request of Governor Jerry Brown, hastening the availability of federal emergency assistance to fire-stricken regions of the state.
The fires have spread with an erratic intensity that has strained resources and kept firefighters struggling to keep up with the flames while catching many residents by surprise.
The remains of some of the Camp Fire victims were found in burned-out vehicles that were overrun by walls of fire as evacuees tried to flee by car in panic, only to be trapped in deadly knots of traffic gridlock on Thursday night.
"It was very scary," Mayor Jody Jones recounted of her family's own harrowing escape from their home as fire raged all around them.
"It took a long time to get out. There was fire on both sides of the car. You could feel the heat coming in through the car," she told CNN. Jones said her family is now living in their mobile home parked in a vacant lot.
Honea said authorities have brought in 13 special search-and-recovery teams to seek out any further victims from the Camp Fire, and have requested additional cadaver-dog crews to assist in the search for human remains.