An age-dependent decline in the amyloid-β (Aβ)-degrading enzyme neprilysin (NEP) has been implicated in the pathogenesis of sporadic Alzheimer's disease (AD). Recently identified risk alleles in the NEP-coding gene further support its role in AD etiology. However, evidence for the impact of NEP on the pathophysiological progression of Aβ plaque formation, particularly in comparison with another Aβ-degrading enzyme, insulin-degrading enzyme (IDE), is still lacking. Furthermore, the functional impact of the NEP mutation, M8V, caused by the AD risk allele in the NEP gene, remains unexplored. Here we found that NEP deficiency in mice accelerates Aβ plaque formation more prominently than IDE deficiency in both male and female mice. Additionally, NEP/IDE double knock-out further exacerbated the plaque deposition of mice, demonstrating a synergistic effect between the two enzymes. We also revealed that the M8V mutation in NEP reduced extracellular Aβ degradation in SH-SY5Y neuroblastoma cells, not by impairing catalytic activity but by increasing phosphorylation at an intracellular serine residue. This alteration in phosphorylation decreases NEP localization on the cell surface and extracellular vesicles, thereby limiting extracellular Aβ degradation. These observations point to the role of aging-associated neprilysin decline in sporadic AD pathogenesis and endorse the strategy of upregulating neprilysin activity to treat preclinical AD.