42CrMo steel is widely utilized in the manufacturing of high-speed, heavy-duty components due to its excellent wear resistance and hardness. To further enhance its performance and extend its service life, ultrasonic rolling strengthening technology has been employed. However, the underlying microscopic strengthening mechanisms induced by ultrasonic deformation require comprehensive investigation. This study aims to analyze the microscopic strengthening mechanisms of unquenched 42CrMo steel through theoretical modeling, processing experiments, and electron backscatter diffraction (EBSD) microstructure characterization. The research focuses on key aspects such as contact mechanics, residual stress distribution, grain boundaries, orientation evolution, and microtexture development under ultrasonic rolling. Experimental results demonstrate that ultrasonic rolling induces severe plastic deformation on the material’s surface, generating significant residual compressive stress within the workpiece. On a microstructural level, ultrasonic rolling increases grain density, refines grain size, and significantly enhances dislocation density. In addition, the formation of fiber texture and a {110}¿<441>¿texture was observed, driven by multi-energy field coupling and the natural rotation of slip planes. Importantly, the high-angle random grain boundaries in the unquenched 42CrMo steel matrix were transformed into low-angle boundaries due to the combined effects of high-frequency vibrations and static pressure, which promoted dislocation slip and redistributed grain orientations. These findings provide an in-depth understanding of the microscopic strengthening mechanisms of ultrasonic rolling, highlighting its potential to achieve precise microstructural control and improve the mechanical performance of 42CrMo steel.