A cell’s ability to survive and to evade cancer is contingent on its ability to retain genomic integrity, which can be seriously compromised when nucleic acid phosphodiester bonds are disrupted. DNA Ligase 1 (LIG1) plays a key role in genome maintenance by sealing single-stranded nicks that are produced during DNA replication and repair. Autosomal recessive mutations in a limited number of individuals have been previously described for this gene. Here we report a homozygous LIG1 mutation (p.A624T), affecting a universally conserved residue, in a patient presenting with leukopenia, neutropenia, lymphopenia, pan-hypogammaglobulinemia, and diminished in vitro response to mitogen stimulation. Patient fibroblasts expressed normal levels of LIG1 protein but exhibited impaired growth, poor viability, high baseline levels of gamma-H2AX foci, and an enhanced susceptibility to DNA-damaging agents. The mutation reduced LIG1 activity by lowering its affinity for magnesium 2.5-fold. Remarkably, it also increased LIG1 fidelity > 50-fold against 3’ end 8-Oxoguanine mismatches, exhibiting a marked reduction in its ability to process such nicks. This is expected to yield increased ss- and dsDNA breaks. Molecular dynamic simulations, and Residue Interaction Network studies, predicted an allosteric effect for this mutation on the protein loops associated with the LIG1 high-fidelity magnesium, as well as on DNA binding within the adenylation domain. These dual alterations of suppressed activity and enhanced fidelity, arising from a single mutation, underscore the mechanistic picture of how a LIG1 defect can lead to severe immunological disease.