The energetics and the electronic structure of AlnC clusters (n=3, 4, 5; 11, 12, 13) have been studied by a global optimization of their geometry without any symmetry constraint. The total energies of these clusters both in neutral and charged states are calculated using an all-electron basis and the generalized gradient approximation to the density functional theory. While Al4C and Al12C clusters share some characteristic features of closed shell structures, namely enhanced stability and low electron affinity compared to their neighboring sizes, their ionization potentials exhibit different behavior. These decrease steadily from Al3C to Al5C while that of Al12C is higher than its neighbors. Carbon is found to form planar structures in small AlnC clusters (n=3, 4, 5) irrespective of their charge state although neutral Al4C possesses a nearly degenerate tetrahedral isomer lying slightly higher in energy from the planar configuration. The results agree well with experimental and previous theoretical data. In larger AlnC (n=11, 12, 13) clusters, carbon occupies an interior site. In Al12C, carbon occupies the center of an icosahedron while it is off-centered in Al11C and Al13C. As an electron is attached, the near degeneracies of the neutral Al4C is lifted whereas nondegenerate isomers of neutral Al12C yield nearly degenerate anions. Both these features produce complicated photoelectron spectra making identification of their adiabatic electron affinity a difficult problem. With the exception of neutral Al12C, the bonding of carbon to aluminum atoms is governed primarily by covalent interaction. The above calculations were also performed with a simplified basis by freezing the atomic cores of aluminum. In most cases, this simple basis yields results in good agreement with all electron calculations.