The victim was hurt by the out of control Japanese akita dog in the town on August 28 last year.
The owner of the dog, 37-year-old Sarah Pantlin, was prosecuted but denied the offence.
Earlier this month magistrates sitting in Northampton found her guilty of owning a dog which was dangerously out of control.
Pantlin, of Grombold Avenue, was fined £180 and ordered to pay compensation of £100.
She must also pay costs of £625 because the case went to a trial and a surcharge to fund victim services of £30.