Nanostructured cellulose was successfully prepared from native cellulose using a homogeneous catalytic H2SO4 hydrolysis pathway in the presence of Cr(III)- and Mn(II)-transition metal salts as the co-catalyst. The effect of transition metal salts with different valence states (Cr3+ and Mn2+) on the physicochemical properties (chemical characteristics, crystallinity index, nano-structure, thermal stability, and morphology) of prepared nanocellulose was investigated. Interestingly, TEM micrographs showed that the Cr(III)-treated and Mn(II)-treated nanocellulose exhibited a web-like nanostructured-surface with average diameters of 44.7 ± 13.2 nm and 58.4 ± 15.3 nm, respectively. XRD study revealed that the crystallinity of nanocellulose was increased because the catalytic degradation of the less crystalline regions of cellulose occurred at a faster rate than its crystalline phases. Cr(III)-treated nanocellulose was capable of rendering a higher crystallinity index (75.6 ± 0.1%) compared with Mn(II)-treated nanocellulose (72.3 ± 0.4%). Furthermore, a dynamic light scattering (DLS) study revealed that Cr(III)-treated nanocellulose showed a smaller distribution range (92% at 14 to 135 nm) compared with Mn(II)-treated nanocellulose (92% at 607 nm). A higher valence state for the Cr(III)-cation, with a trivalent state (+3), rendered a more effective hydrolysis reaction compared with the Mn(II)-cation, with a divalent state (+2), for preparing the nanocellulose.