The remediation of mercury (Hg) contaminated soil and water requires the continuous development of efficient pollutant removal technologies. To solve this problem, a biochar–bentonite composite (CB) was prepared from local millet straw and bentonite using the solution intercalation-composite heating method, and its physical and chemical properties and micromorphology were then studied. The prepared CB and MB (modified biochar) had a maximum adsorption capacity for Hg2+ of 11.722 and 9.152 mg·g−1, respectively, far exceeding the corresponding adsorption value of biochar and bentonite (6.541 and 2.013 mg·g−1, respectively).The adsorption of Hg2+ on the CB was characterized using a kinetic model and an isothermal adsorption line, which revealed that the pseudo-second-order kinetic model and Langmuir isothermal model well represented the adsorption of Hg2+ on the CB, indicating that the adsorption was mainly chemical adsorption of the monolayer. Thermodynamic experiments confirmed that the adsorption process of Hg2+ by the CB was spontaneous and endothermic. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and a thermogravimetric analysis (TGA) showed that after Hg2+ was adsorbed by CB, functional groups, such as the –OH group (or C=O, COO–, C=C) on the CB, induced complexation between Hg and –O–, and part of Hg (ii) was reduced Hg (i), resulting in the formation of single or double tooth complexes of Hg–O– (or Hg–O–Hg). Therefore, the prepared composite (CB) showed potential application as an excellent adsorbent for removing heavy metal Hg2+ from polluted water compared with using any one material alone.