This work presents a comprehensive study on the influence of processing variables (temperature, agitation speed) and type of diisocyanate crosslinker on the rheological properties of liquid and gel-like polyurethane formulations based on lignin and castor oil. With this aim, a green and facile one-step preparation protocol which avoids the use of any harmful catalyst or solvent was proposed. Different processing temperatures (25, 45, 70 °C), stirring speeds (23, 70, 140 rpm) and kinds of diisocyanates (hexamethylene diisocyanate, and toluene diisocyanate) were selected to process the different bio-sourced polyurethane samples. These processing variables have proven to be crucial to modulate and control the rheological (viscous and viscoelastic) properties, and curing kinetics after processing, due to the different chemical structures achieved that were elucidated by means of Fourier-transform infrared spectroscopy and differential scanning calorimetry. In general, a low processing temperature and low stirring speed favour the achievement of gel-like characteristics and/or the formation of highly viscous polyurethanes, as a consequence of a higher number of total hydrogen-bonded carbonyl groups, urethane/urea ratio and ratio of hydrogen-bonded/non-bonded urethane and urea linkages. Besides, the selected one-step process, in comparison with the reported two-step process, provides polyurethanes with similar rheological characteristics by significantly reducing the isocyanate content. Finally, lignin was demonstrated to act as an effective filler agent increasing the values of the viscoelastic moduli.