Preheat grill to low-medium heat.
Season chicken with salt and pepper on both sides.
Place chicken on the grill with smooth side up. Cover and cook for 15-18 minutes for bone-in thighs and 13-15 minutes for boneless thighs.
Flip them over and cook, covered, for another 15-18 minutes. Remove from grill and let it rest for 5-10 minutes before serving.
Place sweet potatoes in a bowl. Drizzle with oil. Toss. Season with salt and pepper.
Arrange potato slices on the grill in a single layer. Cover and cook over medium heat for about 5 minutes or until grill marks appear. Flip over and cook, covered, for another 5 minutes.
Remove from grill. Serve with chicken.