Timothy Cates, 58, was reported missing at around 8 p.m. Wednesday by the staff of Rose Acres Residential Care Facility at 6950 Rose Acres Lane in Felton.
Witnesses reported seeing Cates at about 5:30 p.m. Wednesday near state Highway 9 at Graham Hill Road, according to the sheriff's office.
Investigators have since received new information that Cates was possibly last seen at 2 p.m. Thursday at the corner of Rose Acres Lane and San Lorenzo Way.
Cates was diagnosed with a mental disability and has been living at the care facility for 10 years.
He has been known to frequent businesses in Felton but has always returned to the facility for dinner, officials said.
Cates was described as a white man who is 5 feet 8 inches tall, weighs 160 pounds, and has gray and blonde hair and a long white beard.
He was last seen wearing a blue beanie cap, red raincoat, and blue jeans.
Police have launched a large-scale search and rescue effort, using deputies, volunteers, eight dog teams from Santa Cruz and the Bay Area, and a motorcycle posse.
The California State Parks, California Highway Patrol and Felton Fire Department are also helping with search efforts.
Anyone with information is urged to call 911 or (831) 471-1121.