Lead-free piezoelectric single crystals of undoped and 1 wt% Mn doped 0.80 Na_0.5Bi_0.5TiO₃–0.20 K_0.5Bi_0.5TiO₃ (NKBT) was grown using self-flux. Powder X-ray diffraction analysis revealed that the grown crystals belong to tetragonal system at room temperature. The lattice strain was calculated from Williamson Hall relation for undoped and Mn doped NKBT crystals. A significant change is observed in dielectric behavior of Mn doped NKBT when compared to undoped sample. The diffuseness increased substantially on Mn doped NKBT which masked the ferroelectric to antiferroelectric transition in the dielectric constant plot. The AC impedance study revealed that the conduction is governed by the singly ionized oxygen vacancy. Further, the decrease in the conductivity on Mn doping suggests that Mn replaces the Bi vacancy, which reduces the oxygen vacancy.