A novel highly ordered three dimensional mesoporous graphitic carbon nitride (MGCN-6) with C3N4 stoichiometry having amorphous wall structure, tunable textural parameters with body-centered cubic Ia3d framework has been prepared from 3D mesoporous silica, KIT-6 with adjustable pore diameters as hard templates through a temperature-induced poly condensation followed by the polymerization reaction of cyanamide (CNNH2) precursor. The structure of the resulting mesoporous graphitic carbon nitride materials consists of sheets of three-dimensionally arranged s-heptazine units that are held together by covalent bonds between C and N atoms. The realized MGCN-6 materials have been thoroughly characterized using various techniques such as powder X-ray diffraction (XRD), N-2 physisorption, high-resolution transmission electron microscopy (HRTEM), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), UV-vis and Fourier transform infrared (FT-IR) spectroscopy, and elemental analysis. The characterization data reveal that the resultant materials possess a well-defined ordered three-dimensional porous structure with a high surface area and a large pore volume. The catalytic activity of these materials has been tested in the Knoevenagel condensation between benzaldehyde and malononitrile. This catalyst is found to be highly active and shows a high conversion with 100% product selectivity even at room temperature.