To develop a new utilization approach of biogas residues, mesoporous activated biochar (MB) was prepared from biogas residues and used for adsorption of As(III) from aqueous solution. MB-ZnCl2-600, which was activated with 2.5 M zinc chloride solution at 600 °C, showed superior thermostability and good mesoporous distribution. The total surface area and the mesoporous surface area of the prepared biochar reached up to 892.3 and 701.7 m2/g, respectively, and its pore diameters mainly ranged from 4 to 40 nm. Batch adsorption of As(III) revealed a high theoretic adsorption capacity of 5861 μg/g. The kinetic data revealed good fitting with the Elovich model, while the adsorption data demonstrated excellent correspondence with the Langmuir model. Experimental results confirmed that As(III) adsorption capacity is positively correlated with the mesoporous surface area (R2 = 0.813) of the adsorbent, confirming that mesopores in the MB-ZnCl2-600 adsorbent dominated As(III) adsorption. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy demonstrated that SiO2 provides crucial adsorption sites for the removal of As(III) and that organic functional groups on the biochar surface bonded with As species during adsorption. This study is the first to prepare MB with good mesoporous distribution from low-cost and wide-source starting material of biogas residues, which can be used as a supporting framework for catalysts and other adsorbent applications.