Mangrove pile leftovers which were found as waste at the construction sites were used as a precursor to prepare activated carbon by adopting phosphoric acid (H₃PO₄) as an activating agent. The effects of H₃PO₄ impregnation ratios (3, 4 and 5) and activation temperatures (300, 400 and 500 °C) on the yield, surface area and the adsorption behavior of the prepared activated carbon were investigated. The highest yield of 45% was obtained for the activated carbon prepared at an impregnation ratio of 3 and at an activation temperature of 300 °C (AC3003). Activated carbon prepared at an impregnation ratio of 4 and an activation temperature of 300 °C (AC3004) gave the highest adsorption capacity of 72.3 mg/g at an initial concentration of 150 mg/L. It could be attributed to the high BET surface area of the prepared AC3004 activated carbon which was 1011.8 m2/g. The combined effect of impregnation ratio of 4 and activation temperature of 300 °C managed to produce a mangrove based activated carbon with a high carbon content (94.18%) and successfully form well-developed pore structures. Experimental data of methylene blue (MB) dye adsorption for AC3004 activated carbon was found to fit well to Langmuir isotherm with a correlation coefficient (R2) of 0.9985, to indicate that the adsorption of MB dye on the activated carbon was a monolayer adsorption. The results of the yield and adsorption study showed that the chemical activation method using H₃PO₄ as an activating agent was suitable for developing mangrove pile based activated carbon prepared for the removal of MB dye in an aqueous solution.