The development history of the star sensors used on the Mariner spacecraft is traced, and design and performance details are described. The electrooptically controlled sensor, which was developed for the 1964 Mariner IV Mars mission, was modified for the 1967 Mariner V Venus mission to withstand the intense planetary illumination. The sensor has been further modified for the 1969 Mariner mission to Mars to survive the more severe launch environment and to provide greater capability for automatic search, identification, and tracking. Special star simulation and stray-light test techniques are discussed.