Multi-functionalized magnetic graphene was prepared by surface-initiated initiators for continuous activator regeneration atom transfer radical polymerization (ICAR ATRP) mediated by polydopamine chemistry. The obtained multi-functionalized magnetic graphene was used as an adsorbent for application in adsorption and speciation of arsenic. Adsorption kinetics and isotherms of arsenic were investigated by batch adsorption experiments. The adsorbent displayed simultaneous or selective removal ability for As(III) and As(V) within different pH ranges. The adsorption of both As(III) and As(V) agreed well with the Freundlich isotherm model, and they were effectively adsorbed through chelation and electrostatic interaction, respectively. Moreover, the functionalized magnetic graphene was examined as a column packing material for speciation analysis of arsenic using solid phase extraction technique. As(III) and As(V) could be effectively separated through a sequential elution strategy. An alkaline solution was firstly employed to selectively elute As(V), and then an acidic solution was utilized to elute As(III). The developed adsorbent in the present study was successfully applied to the determination of As(III) and As(V) in natural and environmental water samples with satisfactory results.