The contemporary automotive industry increasingly incorporates composite materials and innovative joining techniques to meet customer demands for lightweight, high-strength alloys. This research explores the feasibility of using friction stir welding (FSW) methods to fabricate dissimilar aluminum nanocomposite lap joints, integrating Al2O3 nanoparticles into the weld nugget. Lap joints were prepared with varying tool rotation speeds (1000–1600 rpm) and the mechanical and metallurgical behaviors of AA6061–AA7075-T6/Al2O3 lap joints were examined. The inclusion of filler material in the weld joint resulted in an 18% increase in shear strength compared to joints without filler. Dynamic crystallization impeded grain boundaries and reduced grain size in the weld stir zone (SZ), with the most enhanced mechanical characteristics observed at a tool rotation speed of 1400 rpm. The shear strength at 1400 rpm was 5780 N, representing a 17% increase compared to joints prepared at 1000 rpm. Field emission scanning electron microscopic examination revealed evenly dispersed Al2O3 nanoparticles within the weld zone, supporting the lap shear test results. Notably, joints created at lower (1000 rpm) and higher tool rotational speeds (1600 rpm) exhibited brittle fracture behavior. The addition of Al2O3 significantly improved lap joint tensile strength due to its uniform dispersion in the SZ. Increasing the FSW tool rotational speed from 1000 to 1400 rpm facilitated nanoparticle dispersion, further enhancing lap joint strength.