Flexible and ultrathin electromagnetic interference (EMI) shielding films are urgently required to manage increasingly serious radiation pollution. In this work, the EMI shielding performance and flexibility of conductive polymer films were addressed by assembling magnetic graphene-based hybrid and cellulose nanofiber (CNF). Briefly, magnetic graphene hybrid anchored by Ni nanoparticles (TRGO@Ni) was synthesized by in situ thermal reduction. Then, highly flexible and ultrathin CNF/TRGO@Ni film with "brick-mortar" layered structure was assembled via a facile vacuum filtration method. As expected, CNF/TRGO@Ni film with 50 wt% filler loading exhibits an enhanced electrical conductivity (262.7 S/m) and EMI shielding effectiveness (32.2 dB) comparing to CNF/TRGO film. Moreover, the excellent mechanical flexibility of CNF/TRGO@Ni film results in that the electrical conductivity and EMI SE only declines by 7.5% after bending 1000 cycles. The EMI shielding mechanism is attributed to the combination of enhancing impedance mismatch, multireflection in "brick-mortar" lamellar structure and endowing synergetic loss by graphene and Ni nanoparticles.