Place the beans in a heatproof container.
In a pot, heat the vinegar and 2 cups water to a simmer. Take off the heat and add the mustard seeds, salt, sugar, red pepper and garlic. Whisk until the salt and sugar dissolve.
Pour the liquid over the green beans. Let cool, and then cover and place in the refrigerator. Allow the beans to pickle 24 hours before using. Pickled green beans will keep tightly covered in the refrigerator for up to 1 month.