Mn${\mathrm{\text{O}}}_2$-modified ${\mathrm{\text{Pb}}}_{0.9625}{\mathrm{\text{Sm}}}_{0.025}{({\mathrm{\text{Mg}}}_{1/3}{\mathrm{\text{Nb}}}_{2/3})}_{0.71}{\mathrm{\text{Ti}}}_{0.29}{\mathrm{\text{O}}}_3$ ceramics were prepared via a solid-state reaction approach. Results of detailed characterizations revealed that the addition of Mn${\mathrm{\text{O}}}_2$ has influence on the grain size, and all samples exhibit a pure perovskite structure. As the content of manganese increases, the volume of tetragonal phase increases. The ceramics with 1.5 mol.% Mn${\mathrm{\text{O}}}_2$ show a high electro-strain of 0.151% at 2 kV/mm. Therefore, this study provides a new insight into the role of Mn${\mathrm{\text{O}}}_2$ addition in tailoring the electrical properties of the Sm-PMN-PT ceramics by acceptor doping.