Combine all ingredients for the mango salsa in a medium-sized bowl. Stir to incorporate. Refrigerate until ready to serve.
Preheat the grill to medium heat (about 400 degrees). Rub olive oil all over the walleye fillet and sprinkle the flesh side with salt and pepper.
Place walleye fillet on the grill, skin side down, and cook for 8-10 minutes, flipping halfway through. My fillet was about an inch thick and took 10 minutes. If you fillet is thinner, adjust the cooking time accordingly!
Once the fillet is done grilling, serve skin side down, garnished with mango salsa.