The woman was inside her Colorado home on Saturday when she heard the screams.
She ran outside to discover a mountain lion on her 5-year-old son, who had been playing with an older brother, according to the Pitkin County Sheriff's Office.
So the woman, whom authorities haven't identified, sprang into action. She "was able to physically remove her son from the mountain lion", and received minor injuries to her hands and legs in the process, the sheriff's office said in a release.
The woman "had to physically remove the lion's paws from her son's face," sheriff's office spokesman Alex Burchetta told People.
AP reported that the woman had to pry his head from the animal's mouth.
Pitkin County sheriff's deputy Michael Buglione said: "She said the mountain lion was on top of her son, crouched down on top of him. She grabbed a paw and lifted it up, and put her right hand in its mouth to pry the boy out of its mouth so the boy could get free."
The animal ran off.
The mountain lion injured the boy's face, head and neck. The boy's father called emergency services while driving his son to hospital.
Aspen Valley Hospital reported the 5-year-old was in fair condition from deep cuts and has since been transported to a hospital in Denver, authorities said. His mother has been released from the hospital, KMGH reported.
AP said she had bite marks on her hand and scratches on her leg.
Mountain lion sightings have increased in Colorado, "likely due to a growing human population encroaching on lion habitat in conjunction with a robust lion population in the state," according to Colorado Parks and Wildlife.
Lion attacks on humans are rare, according to CPW, with "fewer than a dozen fatalities in North America in more than 100 years."
"Most of the attacks were by young lions, perhaps forced out to hunt on their own and not yet living in established areas," according to the agency. "Young lions may key in on easy prey, like pets and small children."
The last known incident in which a lion attacked a human in Colorado was in July 2015, when a young lion attacked a man fishing, according to CPW. That small, yearling male lion was tracked by officials and killed.
After the latest incident, "the mother was able to accurately describe to deputies where the mountain lion had attacked her son," the sheriff's office said. Witnesses reported seeing two lions in the area before the attack.
The mountain lion was estimated to be about 2 years old and not fully grown. Wildlife officials killed two mountain lions in the area within several hours of the attack. Both animals were being examined to determine if they were hungry, diseased or just in the wrong place at the wrong time.
The generally calm and elusive animals are mostly solitary, according to the National Park Service. Males can grow up to 90kg and 2.5m in length, including their tails.
The park service advises people to go in groups when travelling through "lion country," and to keep children close by.
"If you see a lion, stop and do not run. Maintain eye contact and do all you can to appear larger," NPS advises. "Speak calmly to the lion in a firm, calm voice. If attacked, fight back. Lions have been known to be driven away by prey that fights back."
additional reporting AP