We present the first development of a lightweight, monolithic, circularly polarized (CP) K-band horn antenna developed using a novel charge-programmed deposition (CPD) manufacturing process. The horn has a polymer body with metalized interior surface. The CPD process creates complex geometries with programmed charged surfaces via three-dimensional (3D) printing and allows the deposition of metallics anywhere within the 3D layout. Intricate internal structures of the horn, such as the meandered waveguide transition and the septum polarizer, were successfully constructed. Measured patterns of the horn demonstrated great agreement with simulations, with an axial ratio below 2.5 dB in the range of 17.5 - 20.5 GHz. The total weight of the horn is only 11 g, marking a significant weight reduction of around 80% compared to traditionally fabricated horns of similar size and operating frequency. The low-cost and lightweight horn antennas enabled by this process can significantly benefit mass- and cost-sensitive applications such as small satellite and CubeSat missions.