Achieving long-persistent phosphorescence in polymers is a challenge even at low temperatures. In this work, long-persistent phosphorescence (>1 s) is observed in nonconjugated polyurethane derivatives at 77 K, which is among the longest reported phosphorescent lifetimes for an organic polymer. The mechanism for this unusual behavior has been shown by steady-state photophysical characterization and time-resolved emission spectra to arise from the formation of intra- and/or intermolecular carbonyl clusters at low temperature. The lifetime of long-persistent phosphorescence is increased by the introduction of an aromatic monomer into the nonconjugated polyurethane chains. This is attributed to intra- and/or intermolecular n-pi* transitions from electron-rich carbonyl groups to the conjugated aromatic units, thereby enhancing the intersystem crossing rate. Coating an ultraviolet InGaAsN light-emitting diode with a polyurethane derivative as the emitter gives cryogenic afterglow with long-persistent phosphorescence that is observed for up to 7 s with the naked eye.