The purpose of this research was to investigate the effects of cow manure and acacia wood-derived biochar on the immobilization of arsenic (As) and manganese (Mn) in contaminated mine tailings using Mott dwarf Napier grass ( Pennisetum purpureum cv. Mott). Cow manure or acacia wood-derived biochar was separately mixed with mine tailings at rates of 1, 3, and 5% (w/w). Samples of mine tailings and plants were collected every 30 d during the 120-d period. The total As and Mn accumulation amounts in the plants were analyzed in both the underground (roots) and aboveground (stems and leaves) parts of the plants. The results revealed that cow manure and acacia wood-derived biochar can reduce the mobilization of As and Mn in mine tailings and thus reduce their uptake and accumulation in P. purpureum . Acacia wood-derived biochar was able to stabilize and immobilize As and Mn in mine tailings, allowing the metals to be taken up for plant utilization despite the lower plant growth (biomass and relative growth rates) than that obtained with added cow manure. The accumulation amounts of As in the aboveground and underground parts of P. purpureum grown in mine tailings with 5% BC application were 0.52 ± 0.05 mg kg −1 and 1.57 ± 0.1 mg kg −1 , respectively, while the accumulation amounts of As in the aboveground and underground parts were 31 ± 1.08 mg kg −1 and 73.05 ± 2.60 mg kg −1 , respectively. In other words, the percentage reductions in As and Mn uptake and accumulation in the aboveground and underground parts were 78.6% and 63.9% for As and 72.5% and 69.3% for Mn, respectively. The results of this study can be applied for the remediation of heavy metal-contaminated areas, especially gold mines and surrounding areas, as well as in other areas.