Polyurethane (PU) lacquer is a complex synthetic polymer. The mechanical properties of PU lacquers are influenced by the in-service environment, which may affect their function as protective coatings. As of date, limited research work has been published on the mechanical properties of PU lacquer films under environmental ageing conditions. This paper presents an investigation of the mechanical and hygroscopic behaviour of a commercial PU lacquer film. By means of water absorption and desorption tests, the diffusion coefficient and coefficient of hygroscopic expansion were determined. Tensile tests at constant displacement rates and creep tests under various UV/humidity ageing conditions showed a complex dependence of material properties on environmental conditions. A modified Burgers model with environment-dependent parameters is shown to satisfactorily model the mechanical behaviour of the PU lacquer in typical service conditions.