The boy was playing outside his home in Calabasas in the Santa Monica mountains, west of Los Angeles, when the lion pounced on him.
The child was dragged across the front lawn, but his mother ran out and struck the mountain lion with her bare hands until it let go of her son.
The animal was later found and shot dead by wildlife authorities.
The boy suffered injuries to his head and torso, but is now in a stable condition at a hospital in Los Angeles, the Associated Press news agency reports.
