The problem of accessibility to public transit is well-documented in transportation theory and network literature, and is known as the last mile problem. A lack of first and last mile transit services impairs access to public transit causing commuters to opt for private modes of transit over public modes. This paper analyzes the implications of a shared autonomous vehicle (AV) taxi system providing last mile transit services in terms of environmental, cost, and performance metrics. Conventional public transit options and a hypothetical last-mile shared autonomous vehicle (SAV) system are analyzed for transit between Ann Arbor and Detroit Wayne County Airport for life cycle energy, emissions, total travel time, and travel costs. In the case study, energy savings from using public transit options with AV last mile service were as high as 37% when compared to a personal vehicle option. Energy and greenhouse gas burdens were very sensitive to vehicle powertrain and ridership parameters. The results suggest that an AV taxi service providing last-mile transit services could enhance the sustainability of transit by influencing a mode shift from private modes to public modes of transit. Wait times associated with public transit options, as well as high AV technology costs, could be obstacles for a last mile service.