Sulfur may have played a dominant role in controlling the geological history of Mars via five major processes. First, core segregation likely removed up to 10 wt% sulfur, leaving the mantle with 1000’s ppm sulfur. Partial melting of the mantle produced igneous rocks containing sulfides and sulfur-bearing glasses (~1000–2000 ppm sulfur), while releasing sulfurous gases, such as SO2, H2S, and S2, into the atmosphere. These gases would have exerted climate regulation through competing influences of surface warming by greenhouse effects and cooling by aerosols derived from sulfate and elemental sulfur. Prevailing climatic conditions during different epochs controlled sedimentary processing of sulfur, including acidic alteration of basaltic minerals and deposition of sulfates from surface waters. Sulfur was also processed in subsurface hydrothermal systems driven by heat from volcanic provinces or large impacts, the latter also producing sulfur-bearing shock glasses from melting of crustal minerals, as evident in some martian meteorites. The abundance of sulfur on Mars would have provided a ready source for energy and nutrients for sulfur-metabolizing microbes, such as those that dominated the earliest life forms to emerge on Earth. Each type of process affecting sulfur is associated with a typical range of sulfur isotopic fractionation, which is recorded in the isotope ratios of minerals and helps constrain formation and alteration conditions of martian rocks. This contribution discusses the major processes known to involve sulfur on Mars, the evidence for their operation in the past, and their potential role in past martian climate and habitability. We also summarize suggested research targets for future work, including preparation for Mars sample return and desired improvements in space exploration technology for future in situ exploration.