Trim the ham of any tough outer skin and excess fat. Score the top of the ham in a diamond pattern, making shallow cuts about 1" apart. Set the ham, scored side up, in a large bowl. Combine the remaining ingredients in a small bowl. Pour over the ham. Cover, refrigerate and marinate the ham overnight, brushing and basting the top and sides with the maple syrup mixture several times.
Preheat the oven to 325 degrees F. Line a roasting pan with parchment paper. Set the ham in the pan, skin-side up. Keep the marinade refrigerated until needed. Bake, uncovered, 75 minutes. When the ham has baked 75 minutes, baste with 1/3 of the marinade and bake 10 minutes more. Repeat these steps twice more. Transfer the ham to a platter, tent with foil, rest 15 minutes, and then slice and serve.