In this research, the structural and magnetic evolution of the nanostructured alloy Co44Fe19Ni7Zr5Ti10B15 produced by mechanical alloying was investigated. Structural and magnetic properties were examined using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA), and vibrating sample magnetometry VSM). XRD results showed that the iron-based solid solution formed after 40 h of milling. After 120 h of milling, the crystallite size reached its minimum value in this study, about 14.1 nm. Images obtained from SEM showed that with increasing milling time, the particles became more spherical, smaller, and more uniform. The refinement of the crystallite size led to an increase in saturation magnetization to about 113.3 emu/g and a decrease in coercivity to nearly 33.57 Oe for a milling time of 40 h. Using thermodynamic calculations, the final phase composition was predicted. The results showed that since the changes in the chemical enthalpy related to the intermetallic phase are more negative than the changes in the chemical enthalpy related to the amorphous phase and the solid solution, the system has a greater tendency to reach this phase than the other phases. The results of the XRD analysis also confirmed this. Heat treatment was performed on the sample at 200 h at temperatures of 483°C, 534°C, and 633°C. For all three annealed samples, the crystallite size showed an increasing trend and reached its maximum value of 93 nm at a temperature of 633°C. The saturation magnetization reached its maximum value in this study of 124.79 emu/g at a temperature of 534°C, and the coercivity decreased sharply at a temperature of 633°C and reached a value of 111.34 Oe. It was shown that the magnetic behavior of the heat-treated samples depends on the fcc and bcc phase volume fractions in the structure.