The attack occurred around 4:15 p.m. Monday in Whiting Ranch Wilderness Park, the Orange County Fire Authority said.
The boy was grabbed by the neck and his father threw a backpack at the cougar. It let go of the child and grabbed the pack and climbed a tree, OCFA Capt. Tony Bommarito told KTLA.
The county sheriff’s department said it helped evacuate the park and then received permission from state Fish and Wildlife to kill the lion.
Whiting Ranch Wilderness Park sprawls over 2,500 acres (1,012 hectares) of canyons and grasslands in the foothills of the Santa Ana Mountains.
In 2004, a mountain lion killed a lone cyclist in the park and hours later mauled another passing cyclist who survived.