Humic substances play a pivotal role in soil fertility and carbon sequestration, yet the molecular mechanisms governing their formation during vermicomposting of mineral-enriched substrates remain poorly understood. This study elucidates the supramolecular assembly of humic acids (HAs) during vermicomposting of cattle manure supplemented with lignin-rich plant residues (0–25%) and finely ground brown coal (0–15%) as a reactive mineral matrix. A 3×3 factorial design with five replicates was employed, and HA structural evolution was monitored over 60 days using a multi-analytical approach, including solid-state ¹³C nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG-DSC), and scanning electron microscopy (SEM). The coal-enriched substrate (S4: 25% lignin + 15% coal) exhibited the highest HA yield (28.9%), a 55% increase over the control, with a 31% synergistic effect beyond additive predictions. Mechanistically, brown coal acted as a multifunctional scaffold: (i) providing aromatic nuclei that lowered the activation energy for radical-mediated oxidative coupling of lignin-derived phenolics, (ii) stabilizing intermediate polymers via surface adsorption (confirmed by XPS analysis), and (iii) participating in electron-transfer reactions, facilitating redox-mediated condensation. Spectroscopic data revealed enhanced aromaticity (lower E4/E6 ratio of 4.1 vs. 6.2 in control), increased carboxyl functionality, and a 2.3-fold higher aromatic carbon fraction (¹³C NMR). Thermodynamic analysis (TG-DSC) confirmed that HAs from coal-enriched treatments exhibited 18% higher thermal stability, indicating the formation of densely condensed aromatic networks. SEM visualized coal particles as nucleation sites for polymer assembly. These findings provide the first mechanistic evidence that brown coal catalyzes supramolecular humic acid assembly through coupled physical scaffolding and redox activity, offering a framework for engineering carbon-stabilizing organic amendments.