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Microbial reduction of arsenic (As)-bearing Fe(III)-(oxyhydr)oxides is one of the major processes for the release of As in various environmental settings such as acid mine drainage, groundwater, and flooded paddy soil. Pyrogenic carbon has recently been reported to facilitate microbial extracellular reduction of Fe(III)-(oxyhydr)oxides. The aim of this study was to investigate the important hot topic regarding the fate and transformation of As during activated carbon (AC) facilitated microbial reduction of As bearing ferrihydrite. Our results show that the rate and extent of Fe(III) reduction in As bearing ferrihydrite by Shewanella oneidensis MR-1 were accelerated by AC. The AC facilitated reduction caused the release of As(III) into the solution, whereas it caused the preferential immobilization of As(V) on the solid phase. Furthermore, AC accelerated the precipitation of vivianite and siderite in sequence during microbial reduction processes. Both of the formed vivianite and siderite had an insignificant capacity for capturing As(III); however, As(V) was selectively immobilized by vivianite compared to that of siderite. Taken together, our findings provide crucial insights into understanding the role of AC on the redox and immobilization of Fe and As in suboxic and anoxic environments and thus their environmental fate when pyrogenic carbons are employed for agronomic and environmental applications.