Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.
In a large mixing bowl, combine all-purpose flour, whole wheat flour, sugar, baking powder, baking soda, and salt.
In a separate bowl, whisk together the milk, vegetable oil, and egg until well combined.
Pour the wet ingredients into the dry ingredients and mix until just combined; do not overmix.
Gently fold in the mixed berries.
Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full.
Bake in the preheated oven for 20-25 minutes or until a toothpick inserted into the center comes out clean.
Remove from the oven and let the muffins cool in the tin for 5 minutes.
Transfer the muffins to a wire rack to cool completely.
Enjoy warm or store for later!