Incorporation of Ni active species with mesoporous carbon and the resulting composite materials have received much attention due to their nature of flexibility derived from a confluence of metallic and carbonic properties and numerous textural advantages inherited from mesoporous carbon substrates, contributing to their versatility in energy transformation. However, the efficient integration of such composite materials with complementary advantages in properties is still highly desired. Herein, confined growth of Ni(OH)2 nanosheets within three dimensionally ordered mesoporous carbon (3DOMC) by steam-assisted crystallization is propose in this paper. High-crystalline Ni(OH)2 species are found in two forms of confined extra-small nanosheets within the mesopores of 3DOMC and staggered nanosheets over the particles of 3DOMC, which can be reduced to the highly segregative Ni nanoparticles by sufficient reduction. By using as a catalyst for methane decomposition to produce COx-free hydrogen, Ni/3DOMC contributes a maximum CH4 conversion of 66.5% and H2 output rate of 7.4 mmol. gcat−1·min−1 at 850 °C, more superior to the conventional Ni-incorporated active carbon (Ni/AC) catalyst. Moreover, the Ni/3DOMC catalyst also exhibits not only a higher initial reaction rate but also an improved catalytic stability attributed to the abundant homodispersed Ni active species by the highly hybrid structure. It is expected that the flexible component, well-defined structure, and superior CMD performance could promote a great application potential of Ni/3DOMC nanocomposites in catalysis.