In this research, the magnetoelectric and mechanical properties of Cu77–xNi15Sn8Mnx (x = 0, 5, 10, 15, 20 at%) alloy are systematically studied. In the results, it is demonstrated that the magnetic properties of the alloy increase with an increase in the Mn content, as evidenced by the thermomagnetic curve and isothermal magnetization curve. The magnetic properties of the annealed Cu57Ni15Sn8Mn20 alloy reach about 15 emu g−1 at room temperature, and the Curie temperature (Tc) reaches about 360 K in a 1 k Oe magnetic field. The characteristic peak of the magnetic transition in the AC magnetic susceptibility test also corresponds to the Curie temperature. An electrical performance test calculated the resistivity of the alloy, the resistivity of Cu57Ni15Sn8Mn20 in its annealed state is about 26% lower than that of Cu77Ni15Sn8. Furthermore, the hardness test reveals that the Vickers hardness of the Cu57Ni15Sn8Mn20 alloy is about 246.2 HV, demonstrating its excellent mechanical properties. In the research, it is shown that the doping of Mn elements plays a crucial role in the magnetic properties of Cu–Ni–Sn alloys. By adjusting the content of the Mn element, the magnetic performance can be controlled. A valuable approach is provided for enhancing the performance of magnetic Cu alloys.