A family from Woodland Hills, including several adults and a few children, was having a picnic at the Malibu Creek State Park when the attack happened Sunday afternoon, according to a statement from the California Department of Fish and Wildlife.
The children were playing near the group's picnic table when the mountain lion attacked the boy, wildlife officials said. At least one of the adults charged at the lion, who then released the boy and ran up a nearby tree.
When California State Parks rangers and Los Angeles County sheriff's deputies arrived at the scene, the large cat was still in the tree.
After consulting with wildlife officers, they "deemed the mountain lion a threat to public safety and a ranger euthanized it via firearm."
The 5-year-old suffered significant injuries and was airlifted to the Northridge Hospital Medical Center after the attack. He was released Monday morning.
No other injuries were reported.