Mice will eat just about anything, but most mice are primarily herbivore and prefer to eat things like oats, grains, fruits, and seeds. However, they will eat almost anything if they get hungry enough including their own tails.
Some species like the white-foot and grasshopper mouse are omnivorous and eat mostly insects and sometimes other small rodents and are very skilled hunters.
Urban mice (mice that have adapted to live in the city) are not very picky and will eat just about any food source available. Their main food source is found rummaging in the garbage looking for food scraps.
Mice in the wild will typically eat whatever vegetation is available. They will eat whatever fruits and seeds are in season and grows in the area. If they are not avaible or out of season they will eat tree bark, roots, oats, corn, or even potatoes.
Mice that live near farms in the countryside will not only eat food scraps form the garbage, but they will also eat corn, wheat or whatever types of crops are being grown.
You can feed your pet mouse food from the pet store. Make sure the food is safe for mice and make sure it is not meant for hamsters or guinea pigs. You can also supplement their feed with grains like wheat, barley, corn, and oats. The safest and easiest thing is to buy an assortment of grains and seeds formulated for a pet mouse.
Baby mice drink milk from their mother. As they start to grow they will slowly start mixing food in their diet such as corn, wheat, and bread (or mice food you buy in the store if it is a pet) depending on the area and season. The mother will bring her baby mice food home until they are old enough to find food themselves.
Field mice are generally scavengers and are not picky about their diet. They will eat whatever they can find including dead animals and rotten food in the garbage.

A What Do They Eat? project.