Interest in hybrid mechanical systems, in which a mechanical oscillator is coupledto quantum elements such as spins, superconducting circuits, and optical photons, has increased in recent years due to the novel means of controlling and coupling disparate quantum systems that mechanical motion enables. In this dissertation we study the specific system of diamond optomechanical crytals (OMCs), which are capable of hosting and coupling to embedded defect qubits such as nitrogen-vacancy (NV) and siliconvacancy (SiV) center spins. We calculate the expected spin-phonon coupling rate for SiV spins interacting with a diamond OMC mechanical mode and find expected zeropoint couping rates of > 1 MHz. We design, fabricate, and measure diamond OMCs, demonstrating GHz-scale mechanical modes with quality factors > 100,000 at liquid helium temperatures. We also measure nitrogen-vacancy center spins embedded in diamond OMCs and find T2 = 72 μs, comparable to the coherence times of NV spins in bulk diamond with natural 13C abundance.