The Staffordshire Bull Terrier, called Diesel, attacked the 13-year-old and a passer-by who tried to help, while the boy was out walking it in a children's playground in Smethwick.
The dog's owner Kelly Farrell, aged 36, pleaded guilty to allowing it to be dangerously out of control in a public place, during a hearing at Sandwell Magistrates' Court.
The court heard that Farrell had been visiting family from her home in Melbury Lane, Northampton, on November 3 and allowed nephew Nathan Hackett to walk the dog in Londonderry Park, Londonderry Road, before returning home. It was then that the dog, which is now in police custody, attacked.
Prosecuting solicitor Mr Roger Bleazard told magistrates: "One of the victims received life-changing injuries, which I think gives you a flavour of what we are talking of. "It is a serious attack by a dog."
Mr Bleazard said the boy was walking towards the exit of the park's play area with the dog when it grabbed him by the arm. He was punching it in the face to try to fight it off when passer-by Thomas Willetts tried to intervene, himself then being bitten.
Defending, Mrs Dawn Foxall did not enter any mitigating evidence on Farrell's behalf. Sentencing will take place at the same court on January 23.