Highly filled lignin:polyethylene composites, with enhanced mechanical properties, were studied. Composites are prepared using the catalytic grafting technique of polyethylene on lignin particles. This technique improves wetting and dispersion of additives in the matrix as well as interactions between these two components. Mechanical properties (tensile and impact) and morphology of the composites were measured. The experimental results showed that highly filled HDPE/lignin composites (up to 73 vol%) could be produced by catalytic grafting. Tensile properties of the composites were quantitatively analysed using a micromechanical model for particulate composites based on an energy balance. Model predictions for Young’s modulus are in excellent agreement with experimental results. The adjustable parameters of the model were varied from one composite to the other in a way which corresponds to the basic concepts related to particulate composites in general and to catalytic grafting in particular.