Place lamb in a shallow bowl and pour marinade over it. Cover with plastic wrap and marinate for 2-3 hours or overnight, turning the lamb over from time to time.
Set up a covered gas or charcoal grill for indirect cooking.
Remove lamb from bowl and discard marinade. Cook lamb for 45 minutes or until the internal temperature in the thickest part reaches 125-130°F for medium-rare. Thinner parts will be more done. Remove lamb from grill and let rest, covered loosely with foil, for 10-15 minutes before slicing and serving.