The boy was attacked at Whiting Ranch Wilderness Park in Trabuco Canyon and then was rushed to a local hospital, according to the Orange County Fire Authority.
The attack was reported at 4:17 p.m.
Officials say a family of six was walking in the park when the mountain lion attacked and grabbed the child by the neck. The family's father threw a backpack at the animal and it let the boy go and instead grabbed the bag and jumped into a tree.
Law enforcement responded and found the animal in the tree. Because of what was considered a threat to public safety, the animal was killed.
The child is in stable condition.
During the search, sheriff's department deputies had evacuated the park with assistance from a helicopter.