Grape (Vitis vinifera L.) is widely used in juice industries and it contains a variety of polyphenols with antioxidant and anti-inflammatory properties. The effect of packaging materials and storage time on the phenol content and antioxidant stability of grape juices made with a Merlot variety was studied. In order to preserve sensory and nutritional values, the juice was stabilized with a high hydrostatic pressure (HHP) and a polylactic acid (PLA) bottle was tested as a substitute for the polyethylene terephthalate (PET) packaging. Except for o-diphenols, the spectrophotometric assays of the different phenol classes showed higher values in juice after HHP treatment. Overall, a loss of most of the phenols was tested in PLA bottle samples at four months of shelf-life. On the contrary, juices in PET bottle did not show significant changes until the end of storage. A comparable trend was observed also for the antioxidant activity and the individual phenols analyzed by a liquid chromatography-diode array detection-mass spectrometry (HPLC-DAD-MS), which showed anthocyanins as principal class of phenols, followed by flavonols and hydroxycinnamoyl tartrates (HCTs). These results on grape juice stability could be interesting not only for the juice production but also for the development of the best packaging material.