The effect of high doping levels of manganese (Mn) on the structure and electrical properties of (KxNa1−x)NbO3 (KNN) ceramics containing Li, Ta, and Sb has been investigated. The samples were measured using synchrotron X-ray diffraction whereas Rietveld refinement with Fullprof was used to determine the structural information as a function of temperature. Temperature-dependent dielectric measurement was used to compare the phase transition temperatures. The results show that Mn decreases the temperature range of phase coexistence between the orthorhombic and tetragonal phase from ~180°C to ~120°C. The Curie temperature remained unchanged with Mn addition while the dielectric constant and dielectric loss increased with Mn addition. High amounts of Mn led to a reduction in both piezoelectric and ferroelectric properties. The remnant polarization, remnant strain, and piezoelectric coefficient values decreased from 24 μC/cm2, 0.000824, 338 ± 37 pm/V to 13 μC/cm2, 0,00014 and 208 ± 27 pm/V, respectively for the undoped and 5 mol% Mn-doped sample.