In this study, three different technical lignins from different biomass sources (softwood, hardwood, and wheat straw) and isolation processes (kraft, hydrolysis, and organosolv) were used to synthesize liquid lignin polyols by oxyalkylating lignin with propylene carbonate, a safe and environmentally friendly solvent. These polyols were then used to replace 10–30 wt.% of the fossil-fuel-based polyols to formulate flexible polyurethane (PU) foams. It was found that a high molecular weight kraft softwood lignin resulted in a foam with significantly better mechanical properties than foams made with organosolv wheat straw and hydrolysis hardwood lignins. Scanning electron microscopy (SEM) and reaction profile measurements showed that the addition of the lignin polyol decreased foaming times and created foams with smaller cells. Foams produced by replacing 20% of petroleum-based polyols with synthesized kraft softwood lignin polyol passed all the standard requirements for automotive seating applications while containing less isocyanate.