At Mars, charge exchange between solar wind protons and neutral exospheric hydrogen produces energetic neutral atoms (ENAs) that can penetrate into the collisional atmosphere, where they can be converted through collisions into H⁺ and H^–. The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission observed a population of negatively charged particles at low altitudes, whose energies, angular distribution, and dependence on the upstream solar wind were consistent with H^–originating in the solar wind. The highest fluxes of H^– were observed near perihelion and the southern summer solstice. We calculated an average ratio of ~4% between H^– density and H⁺ density, implying a slightly smaller relative abundance than reported previously (~10%). We found that the fraction of H ENAs converted to H^– increases with the solar wind energy, in agreement with laboratory measurements of the H–CO₂ electron capture cross section.