Capillary rise infiltration of molecular glasses into self-assembled layers of rigid nanoparticles (NPs) can produce highly confined molecular nanocomposite films (MNCFs). Here, we investigate the thermal stability and photostability of MNCFs made by confining indomethacin glasses in silica NPs. We demonstrate increasing confinement decreases the rate of thermal degradation and increases the activation energy of degradation (up to ∼70 kJ/mol in 11 nm NPs, ∼3 nm pore size). Upon UV exposure under nitrogen, photodegradation is only observed at the near-surface region of MNCFs, with a thickness of one NP diameter. However, no further degradation is observed, even after prolonged UV exposure. The dramatically improved thermal stability and photostability of MNCFs can be attributed to the slower transport of reaction products, corresponding to the increased Tg (up to ∼30 K in 11 nm NPs). These findings demonstrate that extreme nanoconfinement can prolong the durability of molecular glasses in applications such as coatings and organic electronics.