The paper presents the study of the influence of the process of magneto-abrasive machining (MAM) on the characteristics of the surface layers of flat surfaces of parts made of ferromagnetic material U9 by machining with end-type heads based on high-power permanent magnets that form a magneto-abrasive tool of the "brush" type. For estimation of the influence of the process of magneto-abrasive machining on the surface layer, the parameters of surface hardness were analyzed after the machining of test samples with different powders and under different modes. The degree of influence of the MAM on the surface, both in terms of the hardness of the surface layer and the deformation of samples due to compressive residual stresses arising as a result of machining, was studied. The estimation of the state of the surface layer was performed by the change in hardness after machining, the magnitude of the degree of hardening, to some extent, by the parameters of roughness. The control of changes in internal residual stresses formed in the surface layers of samples due to the interaction of powder particles with the surface during machining was carried out according to the degree of their deformation after MAM. It was found that due to MAM, internal compressive stresses of 30–100 MPa arise in the near-surface layer of the material of the samples, while the magnitude of the stresses varied in inverse dependency, that is, with an increase in the working gap, in the vast majority of cases, a decrease in the magnitude of the stresses was observed. It was shown that the depth of the hardened layer under different machining conditions is up to 200 µm or more, and the strengthening coefficient varies from 10 to 40%.