Lead-free (K0.5Na0.5)(MnxNb1−x)O3 (KNMN-x, x = 0, 0.0025, 0.0050, 0.0075, 0.0100) ferroelectric thin films were prepared by a chemical solution deposition method. The effect of Mn substitution on dielectric, ferroelectric and leakage current properties of KNMN-x thin films was investigated. It was found that 0.5-mol% Mn-doped KNMN-x film can increase the dielectric constant, as well as significantly decrease the leakage current. The KNMN-0.0050 thin film exhibited a low leakage current density of 10−6 A/cm2 at high electric field of 100 kV/cm. As a result, well-saturated ferroelectric P–E hysteresis loop with a large remanent polarization of ∼19.2 μC/cm2 was obtained in the 0.5 mol% Mn-doped KNMN-x film.