It happened on Happy Valley Road near Briones Regional Park in Lafayette.
Authorities spent Wednesday night and Thursday morning searching for Guilliams and his Ford Taurus.
Guilliams was 50 feet down a hillside and hidden by a forest of trees. He sat there silent and trapped inside all night long.
Searchers never found him. But his son did 18 hours after he reported him missing.
"He was sitting upright absolutely still and we called to him and his head moved a little bit," Guilliams' son John Guilliams Jr. said.
He says the winding, narrow stretch of road was unfamiliar to his father, but he took the detour to avoid road work on his way home from the library Wednesday; he was last seen there at 4 pm.
"Overnight and this morning we had a number of patrol vehicles from this area continuing to look for him, unfortunately we were not able to locate him," Contra Costa County Sheriff's spokesperson Jimmy Lee said.
It was not until 11 a.m. Thursday that his son spotted the mangled car -- windows shattered, airbags deployed. The impact was so strong it split a tree right down to the roots.
The 86-year-old man was dazed and bruised.
"I asked him if he was OK or how he was and he said, 'terrible,' and had a smile on his face," Moraga-Orinda Fire Department Captain Vince Matulich said.
"He's been pretty banged up but hopefully he's going to be fine," Guilliams said.
He was taken in a helicopter to John Muir Medical Center in Walnut Creek, where his family remains by his side. He will stay overnight for observation, but the Contra Costa County Sheriff's Department says Guilliams is expected to make a full recovery.