Base excision repair (BER) handles many forms of endogenous DNA damage, and apurinic/apyrimidinic endonuclease 1 (APE1) is central to this process. Deletion of both alleles of APE1 (a.k.a. Apex1) in mice leads to embryonic lethality, and deficiency in cells can promote cell death. Unlike most other BER proteins, APE1 expression is inversely correlated with cellular senescence in primary human fibroblasts. Depletion of APE1 via shRNA induced senescence in normal human BJ fibroblasts, a phenotype that was not seen in counterpart cells expressing telomerase. APE1 knock-down in primary fibroblasts resulted in global DNA damage accumulation, and the induction of p16INK4a and p21WAF1 stress response pathways; the DNA damage response, as assessed by γ-H2AX, was particularly pronounced at telomeres. Conditional knock-out of Apex1 in mice at post-natal day 7/12 resulted in impaired growth, reduced organ size, and increased cellular senescence. The effect of Apex1 deletion at post-natal week 6 was less obvious, other than cellular senescence, until ∼8-months of age, when premature aging characteristics, such as hair loss and impaired wound healing, were seen. Low APE1 expression in patient cancer tissue also correlated with increased senescence. Our results point to a key role for APE1 in regulating cellular senescence and aging features, with telomere status apparently affecting the outcome.