The family was on a hiking trail near the Picchetti Winery in Cupertino, Calif., when the lion grabbed the child.
"He was basically somewhat dragged through part of the brush," Fish and Wildlife Warden Travis Jarrett told KRON-4. "Multiple family members then came to his aid, fought off the attacking mountain lion, then the boy was basically transferred to a local medical facility."
The unidentified boy suffered non-life-threatening injuries to his back and neck, the station reported.
Shawn Ardaiz told NBC Bay Area he was hiking on the same trail.
"The man came down the trail carrying his son, it looked like he had lacerations on the back of his neck... He was bleeding pretty heavily," Ardaiz was quoted as saying. "Originally I thought he had fallen off some rocks, but afterwards we heard a mountain lion attacked."
Mountain lions are known to live in the region and signs on the trail warn of their presence, but attacks in the area are rare and most mountain lions avoid humans.
"We know they're up there. We do get some calls about them, but most of the time, it's 'they crossed the road' or 'went by someone's yard,'" Santa Clara County Sheriff Department spokesman Sgt. Kurtis Stenderup told SFGate.
"If you live in the hills, you kind of expect that," Stenderup was quoted as saying. "But I think it's rare to have a mountain lion attack a person, let alone a child. We're going to look into it and figure out why that whole thing happened."
Wildlife officials told local media that they are using dogs to search for the animal.
While mountain lions engaged in threatening behavior can be trapped and relocated, wildlife officials told the Los Angeles Times that if DNA from a captured animal matches that of saliva on the child's clothing, it would be killed "in the interest of public safety."
The California Department of Fish and Game has more information on mountain lions, and what you should do if threatened or attacked by one.