Heat the oil and garlic in a small skillet over medium heat and cook, swirling the pan occasionally, until the garlic is soft, about 2 minutes. Remove from the heat and pour into a medium heatproof bowl. Add the lime juice, cilantro and 1/2 teaspoon salt, whisk to combine and set aside.
Sprinkle the mahi-mahi generously with salt and pepper. Lightly oil the grill grates. Place the fish on the grill, rounded-side down, and cook until it releases from the grill and has grill marks, about 5 minutes. Carefully flip the fish and continue to cook until firm to the touch, about 5 minutes more.
Divide the fish among 4 plates and spoon some of the garlic-lime oil over top. Serve with lime wedges and cooked white rice and the remaining sauce on the side. Sprinkle with cilantro leaves.
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