In this letter, we report on the role of Mn doping in markedly reducing leakage currents in Ba0.6Sr0.4TiO3 (BST) high-K gate dielectrics utilized in organic thin film transistors (OTFTs) fabricated on plastic substrates. Undoped and 3% Mn-doped BST thin films, deposited by rf magnetron sputtering at room temperature on Pt/Ti/SiO2/Si substrates, exhibited relative dielectric constants of similar to 24-28. At an applied electric field of 250 kV/cm, the 3% Mn-doped BST films exhibited leakage current densities below 2x10(-8) A/cm(2) compared to the much higher value of 5x10(-4) A/cm(2) characteristic of undoped BST films. Pentacene based OTFTs using 3% Mn-doped BST gate dielectrics exhibited low voltage operation of < 10 V. This demonstrates the potential use of Mn-doped BST films as high-K gate dielectrics for stable and low operating voltage OTFTs.