Polymer-derived ceramics (PDCs) are a very attractive class of materials due to their excellent properties such as resistance to high temperatures and harsh environments, adjustability of mechanical and functional behavior, and compatibility with a broad range of shaping techniques. The liquid nature of the silicon-containing polymeric precursors and the composition tuning via addition of fillers set the stage for the creation of functionally graded ceramics (FGCs). Polysilazane-based precursors with and without divinylbenzene (DVB) addition for carbon concentration increase are sequentially cast in three different custom molds. DVB concentration, casting order, and thermal processing conditions are varied to study their influence on the interface nature and composition gradient of the functionally graded ceramic parts which are obtained after pyrolysis at 1000–1400 °C. Synchrotron-based tomographic microscopy was applied for high-resolution 3D visualization of the parts, allowing clear distinction of the dissimilar regions and approximate quantification of the composition gradients tunable from submicrometer to millimeter transition lengths. Achieving spatially defined electrical conductivity contrasts in monolithic silicon carbonitride parts, an LED attached onto a FGC plate outlines a promising use case for harsh environment applications.