The effect of Li and Mn substitution on the dielectric, ferroelectric and piezoelectric properties of lead free K_0.5Na_0.5NbO₃ (KNN) was investigated. Samples were prepared using a conventional solid state reaction method. The sintering temperature for all the samples was 1050°C. The optimum doping concentration for the enhancement of different properties without the introduction of any other co-dopants such as Ti, Sb, and La was investigated. X-ray diffraction analysis confirmed that all the samples crystallize in a single phase perovskite structure. The dielectric properties were investigated as a function of temperature and applied electric field frequency. Compared with Li-substituted KNN (KLNN), Mn-substituted KNN (KMNN) exhibited a higher dielectric constant ɛ_max (i.e., 4840) at its critical transition temperature T_c (i.e., 421°C) along with a lower value of tangent loss at 10 kHz and greater values of saturation polarisation P_s (i.e., 20.14 μC/cm²) and remnant polarisation P_r (i.e., 15.48 μC/cm²). The piezoelectric constant (d₃₃) of KMNN was 178 pC/N, which is comparable to that of lead-based hard ceramics. The results presented herein suggest that B-site or Mn substitution at the optimum concentration results in good enhancement of different properties required for materials used in memory devices and other applications.