There is a significant knowledge gap in the degradation of poly(methyl methacrylate) (pMMA) under conditions experienced by surface coatings in the harsh Australian environment. In the current study pMMA model compounds were exposed to 95 °C temperatures and high UV radiation (1 kW · m−2), separately as well as in combination. Contrary to the findings of previous studies, degradation proceeds in these conditions via a non-radical cyclic mechanism. The mechanism was further confirmed by synthesis and degradation of ethylene-oxide-terminated pMMA, an intermediate product in the cycle. Electrospray ionisation mass spectrometry analysis of thermally degraded samples after 155 weeks shows degradation consistent with the proposed cycle in vinyl-terminated pMMA, while saturated pMMA was shown to be stable after the same period. Saturation delayed the UV-induced degradation, yet these compounds still displayed some slight degradation after 52 weeks, confirming the terminal vinyl bond in pMMA as a weak point. Combined UV and thermal radiation after 56 weeks showed degradation of both pMMA samples, with the vinyl-terminated sample also exhibiting crosslinking. The combination of thermal and UV radiation also caused acceleration of degradation, shown by a comparison of the polymer samples after ≈60 weeks.