The synthesis of one-dimensional nanostructures with specific properties is often hindered by difficulty in tuning the material composition without sacrificing morphology and material quality. Here, we present a simple solid state diffusion method utilizing atomic layer deposition to controllably alter the composition of metal oxide nanowires. This compositional control allows for modification of the optical, electronic, and electrochemical properties of the semiconductor nanowires. Using this method and a novel process for manganese oxide atomic layer deposition, we produced manganese-doped rutile TiO2 nanowires and investigated their structural and photoelectrochemical properties. A homogeneous incorporation of the Mn dopant into the rutile lattice was observed, and the local chemical environment of the Mn was determined using X-ray absorption spectroscopy. The doping process resulted in a tunable enhancement in the electrocatalytic activity for water oxidation, demonstrating that this simple and general method can be used to control the properties of one-dimensional nanostructures for use in a variety of applications including solar-to-fuel generation.