Polyurethane (PU) films were prepared by solution casting using a three-component system, namely, a novel solvolytic lignin, polyethylene glycol (PEG), and tolylene 2,4-diisocyanate (TDI), with dibutyltin dilaurate as a catalyst. An important objective was to incorporate as much lignin as possible. To this end, PU film synthesis was optimized by varying the lignin content (30 to 70 wt.% with respect to PEG), isocyanate-to-hydroxyl (NCO/OH) stoichiometry (0.8, 1, 1.2, 1.5, and 1.8), and PEG molecular weight (400, 600, and 1000). The results showed that the films derived from PEG 600 and a NCO/OH ratio of 1.5 were synthesized with a maximum content of 70% lignin, with respect to PEG. The effects of lignin content on the tensile properties and the thermostability of the PU films were studied. The onset decomposition temperature (TOD) of the lignin PU films reached an average limit of 310 °C, regardless of the lignin content, and 260 °C for a PU film without lignin. Thus, the addition of lignin, as a PEG substitute in polyurethane films, leads to better thermal stability. Furthermore, breaking stress, Young’s modulus, and Shore hardness of PU films increased constantly with lignin content, without reaching a maximum.