Whisk soy sauce, Worcestershire sauce, and garlic together in a bowl; pour into a resealable plastic bag. Add flank steak, coat with marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator, 8 hours to overnight.

Preheat an outdoor grill for medium heat and lightly oil the grate.

Remove flank steak from marinade and shake off excess. Discard remaining marinade.

Cook steak on the grill until slightly pink in the center and beginning to firm, about 7 minutes per side. An instant-read thermometer inserted into the center should read 140 degrees F (60 degrees C) for medium doneness.