Grant McKenzie, 25, was riding around Danestone Country Park on the outskirts of Aberdeen on the bike when he hit Amanda Cardy's collie Ruby.
The impact dislocated the dog's spine and punctured her lungs and she later had to be put down by a vet.
McKenzie initially denied the offence and was due to go on trial facing a string of driving charges at Aberdeen Sheriff Court yesterday.
However, he changed his plea after CCTV footage was reviewed in court.
He pleaded guilty to dangerous driving, driving straight at a pedestrian and then striking a dog causing fatal injuries, and driving while disqualified and with no insurance.
Sheriff Graeme Buchanan deferred sentence until next month for the preparation of reports.