This study investigates the effects of high-pressure torsion (HPT) with varying turns (1/2, 1, 5, and 10) on the microstructure and mechanical properties of as-extruded Mg-0.8Mn (wt%) alloy at ambient temperature. HPT process can reduce the grain size from 3 mu m to 0.4 mu m and induce a texture evolution from a basal fiber orientation to a basal plate configuration with increasing strain. Notably, Mn particles are fragmented and dissolved into the alpha-Mg matrix, indicating enhanced solid solubility of Mn due to HPT deformation. The alloy processed with five HPT turns exhibits remarkable room-temperature ductility, achieving 166 % elongation at a uniaxial tensile strain rate of 1 x 10(- 3) s(- 1). This exceptional ductility is primarily attributed to the activation of grain boundary sliding (GBS) and the non-basal dislocations. During tensile deformation, the fraction of low-angle grain boundaries (LAGBs) decreases, and the average grain size increases from 0.4 mu m to 1 mu m, accompanied by recrystallization growth. This phenomenon can alleviate the stress concentration, and thereby the ductility is improved. These findings underscore the critical roles of GBS and recrystallization growth in enabling a superplastic response during tensile deformation.