Laser micro-grooving has excellent potential to improve the surface properties of metals. In this study, high-speed micro-grooving of the surfaces of mild steel SS400 and aluminum alloy A1050 was investigated using a single-mode continuous-wave fiber laser with an average power of 200 W. Experiments were conducted at laser beam scanning speeds of 1 to 6 m/s. The incident angle of the laser beam in the plane perpendicular to the scanning axis was varied from 0 to 60°. A relatively deep groove (approximately 30 µm in depth) and a high upheaval (10 µm in height) were formed along the scanning line at an incident angle of 45° on the SS400, whereas only shallow grooves were formed on A1050. The micro-groove formation mechanism was discussed on the basis of high-speed camera observation, groove shapes, and the thermophysical properties of the metal. Asymmetric molten metal flow and solidification around the keyhole are thought to play important roles in micro-groove formation. Although the laser irradiation conditions must be optimized according to the thermophysical properties of the material, angled laser irradiation has the potential to effectively form micro-grooves.