A green asymmetric supercapacitor with high energy density has been developed using birnessite-type ultrathin porous MnO2 nanoflowers (UBMNFs) as positive electrode and functional mesoporous carbon nanotubes (FMCNTs) as negative electrode in 1 M Na2SO4 electrolyte. Both of the electrode materials possess excellent electrochemical performances, with high surface areas and narrow pore size distributions. More significantly, the assembled asymmetric supercapacitor with optimal mass ratio can be cycled reversibly in the high-potential range of 0–2.0 V and exhibits an excellent energy density as high as 47.4 W h kg−1, which is much higher than those of symmetric supercapacitors based on UBMNFs//UBMNFs and FMCNTs//FMCNTs supercapacitors. Furthermore, our asymmetric supercapacitor (ASC) device also exhibits a superior cycling stability with 90% retention of the initial specific capacitance after 1000 cycles and stable Coulombic efficiency of ∼98%. These intriguing results exhibit great potential in developing high energy density “green supercapacitors” for practical applications.