Altermagnetism (AM) is a novel collinear antiferromagnetic phase characterized by spin-polarized bands and zero net magnetization. However, realizing AM in monolayer hexagonal lattices is strongly constrained by lattice symmetry. Here, we propose a mirror-symmetry-based design principle and implementation scheme for realizing AM based on monolayer hexagonal lattices and validate it using holey graphyne (HGY) as a representative system. By examining hydrogenated nonmetal units X-H (X = B, C, N, P, and S) introduced along the mirror-symmetry axis and extending the analysis to N-F, we identify the configurations that form two antiparallel spin sublattices related by mirror reflection. The N-H structure forms a stable altermagnetic ground state, with a momentum-dependent spin splitting up to 148.2 meV. The magnetism originates from π-π superexchange interactions, and H/F functionalization stabilizes intrinsic AM by regulating the local charge distribution. Our work offers insights into the design of light-element altermagnetic materials.