The mother heard screams and raced outside the house near Aspen, where she found the animal on top of her son, the Pitkin County Sheriff's Office said.
The boy suffered face, head and neck injuries and was said to be in fair condition at a hospital in Denver.
Officials said two lions were killed.
The unidentified woman said she grabbed one of the animal's paws and stuck her right hand in its mouth to pry it open and free her son's head, Pitkin County Sheriff's Deputy Michael Buglione said.
