Composites of carbon-based fillers and elastomeric matrices are at the heart of developing technologies such as high-strain sensors/actuators and stretchable electronics due to their unique combination of electrical and mechanical properties. Production of these composites typically includes dispersion of filler particles into an uncross-linked polymer matrix such as liquid polydimethylsiloxane (PDMS) and a subsequent cross-linking of that matrix. We show here that, in the cross-linking of PDMS elastomer, carbon-based fillers such as carbon blacks and functionalized graphene can diminish the extent of cross-linking via a deactivation of small molecule catalysts and cross-linking agents. This deactivation is evidenced by the relationship between the filler loading, the composition at which gelation is observed, and the elastomer cure time. We have studied composite mechanical properties over a broad range of cure mixture compositions, and we demonstrate that materials with a high degree of cross-linking can be obtained when corrections are applied for this deactivation effect. Mechanical and electrical properties of these composites are explored with stretchable conductor applications in mind.