Novel magnetic and conductive Ni/carbon aerogels have been successfully fabricated through an autocatalytic reduction process. The resultant Ni/carbon aerogels have nanoporous structure with high specific surface area, low densities, appropriate electrical conductivities and controllable magnetization. Due to these special attributes, the aerogel composites are demonstrated to be a great microwave absorbent possessing strong and controllable EM absorption with ultrathin thickness. Only with filler loading of 10 wt % in the wax, a minimum RL of -57 dB is found at 13.3 GHz with the thickness of 2 mm. Even with the thickness of 1.5 mm, the Ni/carbon aeroge1-3 composite can exhibit the low RLmin value of -32 dB and wide effective EM absorption bandwidth of 4.0 dB. The synergistic effect of the medium dielectric loss, weak magnetic loss and good impedance match, accounts for the high microwave absorption performance.