Combine the marinade ingredients, the margarita mix, juice, and zest of the lime and garlic in a large ziploc bag. Mix to combine. Remove a ½ cup of the marinade, set aside for later.
Add the chicken breasts and seal the bag. Place the bag in a dish (just in case it leaks!) Refrigerate for 2-4 hours.
When you're ready to grill, heat the grill to 350 degrees.
Remove the chicken from the bag, shaking off excess marinade. Discard used marinade. Season the chicken with salt and pepper.
Grill, covered, 5-7 minutes on each side, frequently basting with reserved marinade. (Grilling time will vary based on the thickness of the chicken.)
Chicken is done when an instant-read thermometer reads 165°. Remove from grill, rest on a clean plate covered with foil for 5-10 minutes before slicing.