Put the trimmed chicken thighs into a large zip-top bag or airtight container.
Combine the olive oil, minced garlic, and chopped cilantro in a blender or food processor and blend until smooth.
Pour the cilantro-garlic marinade into the bag with the chicken, remove excess air, seal, and knead the bag to coat the chicken evenly.
Marinate the chicken in the refrigerator overnight or all day for best flavor.
Preheat the grill to medium heat and oil the grates or spray with nonstick spray.
Remove chicken from the marinade and place on the grill. Grill 4 to 5 minutes on the first side, until good grill marks appear.
Flip the chicken and grill an additional 3 to 4 minutes, or until cooked through (no pink in the center and juices run clear).
Transfer the cooked chicken to a platter and brush the sweet chili sauce over the top before serving.
