The effects of spin-polarized electron and hole injection from ferromagnetic contacts on the formation and distribution of singlet and triplet excitons in a conjugated organic semiconductor are modeled. Electron and hole transport in the semiconductor are described by spin-dependent device equations for a structure resembling an organic light emitting diode. The formation of electron-hole pairs at a given site is modeled as a Langevin process, and the subsequent local relaxation into the lowest energy exciton states is described by rate equations. Once formed, excitons may recombine in the semiconductor or diffuse through the material and recombine at the contact interfaces. The model calculations yield steady-state spatial profiles for singlet and triplet excitons. It is shown that spin-polarized injection increases the formation of singlet excitons, and that the diffusion of excitons has significant effects on the triplet exciton profile.