We investigate how the Al content in Alx(Mn0.76Co0.24)1-x (x = 0.45, 0.50, 0.55) alloys and the Mn/Co ratio in Al0.50MnyCo0.50-y (y = 0.36, 0.38, 0.40) alloys affect the magnetic properties. Structural and magnetic investigations by experiments, Thermo-Calc, and ab initio calculations show a dual-phase microstructure containing different fractions of a paramagnetic body-centered cubic (BCC) solid solution and a ferromagnetic B2 phase. The BCC/B2 phase fraction is sensitive to the Al content which strongly affects the saturation magnetization, magnetic transition temperature, and magnetocaloric properties. The magnetocaloric properties of the Al0.50Mn0.38Co0.12 alloy show peak values around 430 K with a magnetic entropy change of 0.71 Jkg−1K−1, an adiabatic temperature change of 0.40 K, and a refrigeration capacity of 25.56 Jkg−1 under a magnetic field of 650 kAm−1 (0.82 T). These results open new possibilities for identifying promising medium-entropy alloys for magnetocaloric applications.