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 degrees C to 120 degrees 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 mu C/cm2, 0.000824, 338 +/- 37pm/V to 13 mu C/cm2, 0,00014 and 208 +/- 27pm/V, respectively for the undoped and 5mol% Mn-doped sample.