While attempting to confine Fe(Htrz)3 (Htrz = 1,2,4,-1H-triazole) into a mesoporous silica matrix during its formation by electrochemically assisted self-assembly (EASA), we have discovered that such spin crossover complex is likely to act as the template (in place of the surfactant species) to form in one step a composite mesoporous material (Fe(Htrz)3@SiO2). The EASA method usually leads to the vertical growth of mesoporous silica thin films owing to the electro-induced condensation of silica precursors (i.e., tetraethoxysilane, TEOS) around tubular micelles (i.e., made of cetyltrimethylammonium bromide, CTAB) oriented orthogonally to the underlying support. In the presence of Fe(Htrz)3 in the starting sol (in addition to TEOS and CTAB), two distinct situations can be reached. At low Fe(Htrz)3 concentration (≤ 3 mM), the vertically aligned mesostructure is formed and Fe(Htrz)3 complexes are incorporated along with the surfactant phase, but most of them are released upon surfactant removal. At high Fe(Htrz)3 concentration (typically 5 mM), a wormlike mesoporous film is obtained in which Fe(Htrz)3 species act as a real template for the formation of a mesoporous Fe(Htrz)3@SiO2 film. Interestingly, the iron-triazole 2 complex is strongly entrapped in the silica matrix as it cannot be removed by solvent extraction (contrary to CTAB), as evidenced from X-ray photoelectron spectroscopy (XPS). Even more attractive are the electrochemical properties of the composite Fe(Htrz)3@SiO2 material, exhibiting highly stable operational stability (i.e., identical voltammetric signals upon multiple successive measurements), contrary to Fe(Htrz)3 species incorporated by impregnation of the surfactant phase which are found to leach out in solution upon use. Such in situ elaborated mesoporous composite made of Fe(Htrz)3 template strongly immobilized in a silica host is thus promising for bridging the gap between soft and hard functional organic-inorganic materials, which is briefly illustrated here for the mediated detection of hydrogen peroxide thanks to the electrocatalytic properties of the Fe(Htrz)3 complex, which are maintained even when immobilized in the composite Fe(Htrz)3@SiO2 film.