Combine mayonnaise, garlic, salt, paprika, and Sriracha in a bowl and mix well.
Add the mixture to a gallon freezer bag and then place the leg of lamb into the freezer bag and seal tightly. Move the meat around in the bag to ensure it is fully overed in the marinade.
Place the bag in n refrigerator to marinate for at least 30 minutes, up to 4 hours.
Preheat the grill to 400 degrees F. Remove the lamb from the bag and place it on the heated grill. Sear each side for 5 minutes.
Close grill lid and cook for 30-40 minutes or until the lamb reaches an internal temperature of 145 degrees F.
Remove from the grill and let rest for 10 minutes.
Carve the leg of lamb against the grain into thin slices, ensuring each piece showcases the succulent pink center. Serve immediately.