Deputies were called out to investigate reports of sightings 35 times in 2010, compared to 14 times in 2009, sheriff's spokesman Lt. Ray Lunny said.
The majority of the sightings were reported in communities like Woodside, La Honda, Portola Valley and Emerald Hills that overlap with large tracts of forested foothills including the San Francisco State Fish and Game Refuge and other open spaces.
However, a spike in sightings doesn't necessarily mean that more of the cats are roaming around.
"A lot of alleged sightings turn out not to be mountain lions at all," state Department of Fish and Game spokeswoman Dana Michaels said. "We've had bobcats, dogs, even domestic housecats be mistaken for mountain lions."
The Department of Fish and Game is typically contacted by the sheriff's office whenever a mountain lion sighting is reported, but field biologists in the area aren't able to investigate every report.
Department biologist Kasie Barnes said there are no official data showing that the number of mountain lions has increased in San Mateo County. In fact, she said, the most recent statistics show that the opposite is true statewide.
"What we have now shows that numbers are decreasing overall," Barnes said.
Michaels said media coverage of "high profile" encounters could explain the increase in reported sightings.
"When one sighting is reported, there tend to be a lot more right after that," Michaels said. "Naturally, we get a little more attentive."
According to the department, which logs an average of 400 mountain lion sightings annually in California, only 3 percent of reports result in a mountain lion being identified as an imminent threat to public safety.
Of the 35 sightings recorded in San Mateo County by the sheriff's office in 2010, DFG officials were called to 16 and verified the presence of mountain lions just once.
In February, two brothers hiking in Pescadero Creek County Park reported fighting off two aggressive mountain lions by shouting and swinging a stick in a standoff that lasted several minutes.
Game wardens confirmed the report and deemed both cats a threat to public safety.
The park was shut down for three days while wardens, a professional tracker and trained hounds attempted to hunt and kill the aggressive cats.
They were not located, and the park reopened once officials announced they were confident the animals had moved beyond its boundaries.
Thirteen mountain lion sightings were reported to the sheriff's office in the following three months, compared to just three during the same time period in 2009.
The sheriff's office, which has been keeping a record of mountain lion sightings since mid-2008 due to media interest, will continue to respond to sightings in case one ever poses a threat, Lunny said.
Tips on how to discourage mountain lions from approaching homes, families, pets and livestock can be found at the Department of Fish and Game's website www.dfg.ca.gov/keepmewild.