Rinse mahi mahi fillets and pat dry with paper towels.
Prepare a marinade by stirring lemon juice, olive oil, oregano, spearmint, lemon zest, garlic, and salt together in a baking dish just big enough to accommodate the fillets without overlapping. Lay the mahi mahi fillets into the marinade.
Marinate fish in the refrigerator for 30 minutes, turn, and continue marinating another 30 minutes.
Preheat an outdoor grill for high heat and lightly oil the grate.
Remove fillets from marinade and shake to remove excess. Discard remaining marinade.
Cook fillets on hot grill, but not directly over flame, to seal in moisture, about 15 seconds per side. Reduce heat to medium and continue cooking fish away from flame until the flesh flakes easily with a fork, 12 to 15 minutes.
