This study aimed to produce activated biochars (BCs) from Moroccan algae residue (AG) and olive pomace (OP) using mechanochemical activation with NaOH and ball milling (BM) for treating artificial textile wastewater containing methylene blue (MeB). The produced OP-activated BC by BM showed the highest absolute value of ζ-potential (-59.7 mV) and high removal efficiency of MeB compared to other activated BCs. The nonlinear pseudo-first-order kinetic model was the most suitable model to describe the kinetics of adsorption of MeB onto biochars produced from AG and the NaOH-activated BC from OP, whereas the nonlinear pseudo-second-order kinetic model suits the OP raw biochar and BM-activated BC. The nonlinear Langmuir isotherm model was the most suitable model for describing MeB adsorption onto BCs, compared to the nonlinear Freundlich isotherm model. The maximum adsorption capacities of AG-activated BCs with NaOH and BM were 13.1 and 9.1 mg/g, respectively, while those of OP-activated BCs were 2.6 and 31.8 mg/g, respectively. The thermodynamic study indicates the spontaneous and endothermic nature of the adsorption process of most activated BCs. In addition, Δ° values indicate the increase of randomness at the solid-liquid interface during MeB sorption onto BC.