A family group from Woodland Hills, consisting of several adults and a few children, was having a picnic at the 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.
"The adults were able to respond. They scared off the mountain lion," California Department of Fish and Wildlife spokesperson Steve Gonzalez said.
When California State Parks rangers and L.A. 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."
"It was deemed a public safety issue. Anytime an animal - a wildlife animal - attacks a human, the animal is euthanized," Gonzalez said.
The 5-year-old suffered significant injuries and was airlifted to the Northridge Hospital Medical Center after the attack. He was released Monday morning.
Gonzalez said the boy is expected to make a full recovery. No other injuries were reported.
Mountain lion encounters are rare, especially during daylight hours. The big cats usually avoid human contact and do most of their hunting at night. Experts say the odds of another cougar attack are low.