Nb and Mn were doped, respectively, to 0.8Pb(Sn0.46Ti0.54)O3–0.2Pb(Mg1/3Nb2/3)O3 (PST–PMN) to improve electrical properties for meeting the requirement in various fields. The additions of Nb and Mn influence in a pronounced way the structure, and improve the densities of the ceramics. Nb-doped PST–PMN increased the piezoelectric coefficient d33, planar electromechanical coupling kp, and relative dielectric constant ε, indicating “soft” piezoelectric behavior. Mn doping played a “hard” part, which significantly enhanced the mechanical quality factor Qm without deteriorating other piezoelectric properties. The most excellent properties of Nb-doped PST–PMN were obtained with doping amount of 0.75 mol%, specifically d33, kp, being on the order of 455 pC/N, 57.5% and 3560, respectively. The addition of 0.75 mol% Mn for PST–PMN presented the optimum electrical properties, with Qm of 554, d33 of 430 pC/N, kp of 57.0%, ε of 2770. It was proposed that the addition of Nb, Mn generated different defect dipoles involved in the domain walls motion and intrinsic piezoelectric responses, leading to different effects on electrical properties.