Start by making the pasta. Bring salted water to a boil, add the pasta, and cook according to package directions until al dente. Set the cooked pasta aside to cool before putting the rest of the salad together.
While waiting for the pasta to cool, prepare the homemade balsamic dressing.
Put together the mason jar salads. Pour the dressing into each mason jar and then layer in the tomatoes, mozzarella, pasta, and spinach.
Place the lid tightly on the jars. Store the spinach pasta salad jars in the refrigerator.
Shake the salad and pour it into a large bowl when you're ready to eat.