Highly efficient and cost-competitive dissolution of lignin is vital to value-added utilization of lignin waste streams. Here, a novel and cost-efficient supramolecular eutectic liquid (phenol-MAH) comprising phenol and maleic anhydride was developed, which can rapidly dissolve lignin with ultrahigh-solid loading (>50 wt%) at room temperature. About 70 wt% lignin loading in phenol-MAH leads to a highly cohesive plasticine state of lignin, which is unprecedented in lignin-based materials. Experimental and computational approaches revealed that the cooperation of strong π-stacking and hydrogen-bonding interactions is responsible for the extraordinary solution-processability of lignin in phenol-MAH. Enhanced compatibility and activity of lignin are also ascribed to the phenolation and acylation of phenol-MAH. Furthermore, phenol-MAH was applied to develop renewable lignin-based dielectric films without solvent removal. The resulting films with a nano-network structure display highly tunable dielectric properties attributed to α-dispersion and the Maxwell–Wagner effect. The modulation of the crosslinking density leads to controllable mechanical properties (e.g. yield stress, 0.05–31.50 MPa) of the films by changing lignin loading. The tailored performance demonstrates that the bio-film can be used as a renewable dielectric material for transient electronics. The scalable phenol-MAH strategy is expected to facilitate the fabrication of novel functional bio-materials with high lignin-loading.