He now faces up to six months behind bars.
Sean Deakin, who had owned the dog only for a few days, chased the male Staffordshire bull terrier, named Tyson, around the kitchen, trapped it between his legs and stabbed in it the chest with a six-inch knife he had grabbed from a drawer, the Daily Mail reported.
The boy left the dog dying on the kitchen floor of his flat while he went to the JobCentre to sign on.
When he returned home, he found the dog lying immobile and vomiting. The dog took eight agonising hours to die.
A witness to the attack alerted Royal Society for the Prevention of Cruelty to Animals (RSPCA), a charity that promotes animal welfare, who found Tyson's body in a wheelie bin outside Deakin's home four days later.
Deakin was arrested and charged with three counts of causing unnecessary suffering to an animal.
He denied the offences but was found guilty at Manchester magistrates court and now faces up to six months behind bars.
The court heard Deakin and his girlfriend Sarah Tame, 19, had taken ownership of Tyson just days before the attack after answering an advert on classifieds website Gumtree in May last year.
Describing the crime as henious, Inspector Danielle Jennings, of the RSPCA, said the attack was the worst case she had ever encountered.
"This was an absolutely heinous crime against an innocent animal who had done nothing wrong other than suffer the misfortune of being taken into that flat," she said.
"In four years as an inspector this is without a doubt the worst attack on an animal I have encountered," she said.
Chairman of the Bench, Hugh Keachie told Deakin the starting point for such crimes was a custodial sentence.
He said: "As a bench this offence absolutely appalled us and we need to make sure you understand how serious this is."
Sarah Tame pleaded guilty to one count of causing unnecessary suffering to an animal at an earlier hearing for not seeking veterinary attention for Tyson.
