To decrease the eutrophication caused by nitrogen (N) and phosphorus (P) in water, magnesium-modified corn stalk biochar (MgB) was prepared under the synergistic impact of the multi-pyrolysis temperatures and Mg²⁺ contents for the co-adsorption of ammonium (NH₄⁺-N) and phosphate (PO₄³⁻). The co-adsorption mechanism, slow-release performance and plant application of MgB were systematically studied. The results showed that pyrolysis temperatures (350–650 °C) and Mg²⁺(0–3.6 g/L) contents not only altered the physicochemical properties of biochar, but also significantly affected the adsorption efficacy of MgB. The adsorption of NH₄⁺-N and PO₄³⁻ was in accordance with Langmuir–Freundlich and pseudo-second-order kinetic models (Q_max = 37.72 and 73.29 mg/g, respectively). Based on the characteristics, adsorption kinetics and isotherms results, the adsorption mechanism was determined and found to mainly involve struvite precipitation, ion exchange, and surface precipitation or electrostatic attraction. Compared with the leaching performance of chemical fertilizers (CF), after adsorption of NH₄⁺-N and PO₄³⁻ (MgB-A), MgB had a more stable pH and lower conductivity. Leaching of NH₄⁺-N and PO₄³⁻ by MgB-A was controlled by both the diffusion mechanism and the dissolution rate of struvite and Mg-P. The excellent long-term slow-release performance and abundant Mg²⁺ of MgB-A promoted the growth of Zea mays L. and Lolium perenne L. Overall, this study suggested that MgB could realize a win–win outcome of struvite biochar-based fertiliser production and wastewater treatment.