Multifunctional mesoporous solid acids were prepared by the sulfonation of carbonized de-oiled seed waste cake (DOWC), a solid waste from biodiesel production. Detailed structural characterization of the materials by elemental analysis, FT-IR, Raman, XRD, XPS, TGA, NH3-TPD and N2-physisorption showed that they were structurally different from the carbohydrate and resin based sulfonated carbon catalysts. In addition to the typical single bondOH, single bondCOOH and single bondSO3H groups they contain several N species (pyridinic, pyrrolic etc.) incorporated in their carbon frameworks. The basic structural unit of these materials is a flexible carbon nitride sheet which is extensively functionalized with acidic groups. Our results show distinct effects of raw material composition and preparation methods (activation, sulfonating agent etc.) on structure, stability, surface acidity and textural properties. Here, catalyst single bondSO3H density and porosity (pore size, pore volume and surface area) had a direct effect on activity. Also, H2SO4 was less useful than 4-BDS (4-benzenediazoniumsulfoante) as a sulfonating agent. The best catalysts with mesoporous structure (average pore diameter 3.9–4.8 nm, pore volume 0.28–0.46 cm3 g−1) and single bondSO3H density (0.70–0.84 mmol/gcat) were obtained by 4-BDS sulfonation of chemically activated DOWCs. In contrast, hydrothermal H2SO4 sulfonation of DOWC produced a non-porous catalyst with high single bondSO3H density while those obtained by H2SO4 treatment of activated biomass (AC) had a porous structure with low single bondSO3H density (0.19 mmol/gcat). Furthermore, the reported catalysts show excellent activity in two reactions of interest in biomass conversion: cellulose saccharification (glucose yield 35–53%) and fatty acid esterification (conversion upto 97%) outperforming H2SO4, conventional solid acids (zeolites, ion-exchange resins etc.) as well as sulfonated carbons reported earlier works, confirming their potential as alternative environmentally benign solid catalysts for sustainable, carbon efficient biorefining.