The effect of C substitution in the AlLiB14 lattice is examined using first-principles methods. The inter-icosahedra B site is found to be the most favorable B site for C substitution and the formation energy is predicted to be 1.7 eV in B-rich conditions. Substituting C does not affect the band gap, nor does it introduce defect states to the gap. An ideal brittle cleavage model is used to study the impact of C doping on the mechanical properties of AlLiB14, and it is concluded that introducing C to the crystal decreases the ideal fracture strength by 3.3 GPa, which is about a 12% reduction in overall strength.