High-quality lead-free piezoelectric K_0.5Na_0.5Nb_1−xMn_xO_3−δ (KNNMx, 0 ≤ x ≤ 0.10) films have been successfully deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by a modified sol–gel method. The effects of Mn substitution on the microstructure, morphology, lattice vibrations, and optical and ferroelectric properties of the KNNMx films have been investigated in detail. All films are polycrystalline, crack-free and show a pseudo-cubic (pc) structure with a thickness of about 215 nm. Raman analysis indicates that the characteristic frequency of ν₁, ν₅ and ν₁ + ν₅ modes shifts towards lower wavenumbers with increasing Mn concentration. The optimal ferroelectric properties were obtained in the film doped with x = 0.06, whose remnant polarization (2P_r) and coercive field (2E_c) values at the applied electric field of 1000 kV cm⁻¹ are 51 μC cm⁻² and 265 kV cm⁻¹, respectively. The increased valence of Mn²⁺, which is substituted at the Nb⁵⁺ site as Mn³⁺, plays an important role in reducing the amount of both oxygen vacancies and holes. In addition, the dielectric functions of the KNNMx films have been uniquely extracted by fitting ellipsometric spectra with the Adachi dielectric function model and a four-phase layered model (air/surface rough layer/KNNMx/Pt) in the photon energy range of 1.5–5.5 eV. The optical band gap (E_g) slightly decreases, while the high-frequency dielectric constant (ε_∞) linearly increases with increasing Mn concentration. Moreover, temperature dependent optical dispersion behavior of the KNNM0.06 film has been investigated from 300 K to 800 K. The analysis of E_g and the extinction coefficient (κ) reveals the correlation between optical properties and structural phase transition. Furthermore, a distinct in-plane (180°) polar nano-domain pattern with a well-defined rectangular phase hysteresis loop has been observed in the KNNM0.06 film from piezoresponse force microscopy (PFM) experiments. The present results could be crucial for potential multifunctional KNN-based device applications.