Sean Janas pleaded guilty in Marathon County to mistreating an animal resulting in death and intentionally poisoning an animal.
Janas entered a plea deal Tuesday with prosecutors. As part of the deal, charges of obstructing an officer will be dismissed as well as an unrelated theft case from Washington County.
Prosecutors say Janas poured bleach and Drano down the throat of a German shepherd-Labrador mix named Mary who died in June 2012.
Prosecutors also say Janas kept a diary detailing how she hated the dog and saying she hoped it died.
Assistant District Attorney Theresa Wetzsteon tells WSAW-TV prosecutors will seek the maximum punishment when Janas is sentenced March 17.