We show that carbon morphology, its distribution and bonding topology, affects the mechanical properties of SiCO ceramics. Such SiCO ceramics consist of a SiCO glass matrix and an embedded “free” carbon phase. SiCO models are generated with identical composition but different morphologies of the “free” carbon phase through molecular dynamics simulations using a Tersoff potential. Subsequently, we compute elastic properties and analyze the impact of morphology, composition, and density. A key outcome is that models with identical composition and density, but different “free” carbon morphologies, exhibit different shear moduli and Young’s moduli. “Free” carbon morphologies are tailored through annealing temperatures and holding times and vary from dispersed small units to graphene-like segregations and tubular structures within a SiCO glass matrix. Therefore, a “rule-of-mixture” based on composition or volume content does not correctly predict mechanical properties in SiCO, and instead morphologies of the constituents and their mutual interaction must be considered.