The 18-month-old dog stepped between the big cat and the youngster during a dramatic encounter in western Canada.
Police, who later shot the cougar dead, credited Angel for saving Austin Foreman's life.
He had gone to retrieve some firewood outside his family's home in Boston Bar, about 150 miles north-east of Vancouver.
Who's a good dog: Angel the golden retriever took on the mountain lion after it attacked an 11-year-old boyThat's when the mountain lion charged across the yard at the boy.
'I was really scared. At first, I didn't know it was a cougar. I thought it was another dog,' Austin told CTV News on Sunday.
'As soon as it went underneath the light, I saw that it was a cougar. I knew at that moment, I had to go inside.'
Angel, who had been at Austin's side, attacked the big cat.
Austin ran into the house and screamed: 'A cougar is eating Angel!'
Mrs Forman said she had to get Austin to repeat himself a few times because he was so shaken.
When she looked out the window, Mrs Foreman couldn't see anything but she could hear Angel wimpering.
Saved by his dog: Austin Forman speaks with reporters as his mother looks on
A Royal Canadian Mounted Police officer shot the mountain lion dead as it attacked Angel'To feel so helpless, I knew I couldn't do anything,' she said.
She called her father-in-law, who told her to call the police.
Royal Canadian Mountain Police Constable Chad Gravelle was nearby and was at the scene within a minute.
The officer found the cougar under the back porch and heard the dog cry out as the cougar chewed its neck.
He fired two rounds into the cougar's rear end, but the cat continued its attack.
The officer closed to within five feet and shot the animal again, killing it.
Even after it was dead, the cat's jaws were clenched on Angel's face.
A mountain lion like the one that attacked the golden retrieverAngel was silent for a few moments but then took in a big gasp of air and got up.
Austin Forman said the dog who was recovering at a local vet's has numerous puncture wounds around her head and neck, as well as a swollen eye.
Mrs Forman said before the attack, her son had come into the house to tell the family how cute it was that Angel had been following him around.
'She must've known something was up,' she said.
Last week, when Angel was frolicking in the snow, family members called her a 'snow angel,' the mother said. 'Now, she's our guardian angel.'