In a small bowl, mix the mustard, oil and garlic together.
Place the chicken in a Ziploc bag, or a plastic container with a lid. Pour the mustard marinade over the chicken and let it marinate in the fridge overnight, or at least for 6 hours.
Preheat the the grill to 400 degrees Fahrenheit. Place chicken on on the grill and cook for about 6-10 minutes on each side, or until the internal temperature reaches 165 degrees F.