<p>Stress-strength reliability is a fundamental concept in reliability theory that quantifies the probability that a system’s inherent strength surpasses the stress it experiences. In many real-world contexts, such as reliability assessment of failed mechanical components, forensic investigations, and retrospective medical analyses, evaluating stress-strength reliability based on past lifetimes is not only relevant but essential. This paper proposes a novel framework for assessing stress-strength reliability in the context of past lifetimes using a quantile-based approach. Theoretical properties of the proposed measure, including its bounds and behaviour under monotonic transformations, are examined. A nonparametric estimator for the quantile-based past stress-strength reliability is developed, and its performance is evaluated through simulation studies. In addition, a real data example is provided to illustrate the applicability of the proposed approach.</p>