In the title mol-ecule, C(21)H(18)N(2), the mean plane of the benzene ring of the mesityl group forms a dihedral angle of 82.69 (4)° with that of the phenanthroline ring system. The crystal structure is stabilized by π-π stacking inter-actions between the phenanthroline system and the benzene ring of the mesityl group of a symmetry-related mol-ecule, with centroid-centroid distances of 3.7776 (14) and 3.7155 (13) Å.