If you don't want mice in your house then you need a snake in your yard.
This is a Northern Watersnake that we saw out in the marsh in Rondeau Bay. It is about a metre long.
Nerodia sipedon sipedon
The northern watersnake can be found in and around almost any permanent body of fresh water within its range, including lakes, rivers and wetlands. Rarely far from shoreline habitats, these snakes can be found in shoreline vegetation, basking on rocks and logs, or in other open habitats along the edges of the water or under rocks along the shoreline. Northern watersnakes hibernate underground in dens or crevices, or in beaver lodges.
source - Ontario Nature.