Present-day climate change has altered the phenology (the timing of periodic life cycle events) of many plant and animal populations worldwide with varying concomitant effects on population fitness. Representing short-term responses to an altered weather regime, these changes are largely explained by phenotypic plasticity. However, for long-term persistence under climate change, evolutionary changes in phenological traits are likely to be needed, requiring among-individual genetic variation. In this study we use a large 15-year dataset on an ectotherm model, the Swedish sand lizard (Lacerta agilis), to investigate how inter-annual temperature variation affects oviposition date in a high latitude ectotherm, a trait strongly linked to offspring fitness and survival. With an interest in both short- and potential long-term consequences, a mixed-model random regression analysis was applied, yielding estimates of population plasticity and among-individual variation in the average, as well as the plastic, response to temperature. Our results indicate that climate warming is likely to have positive fitness effects in this lizard population through an advancement of oviposition date. This prediction is consistent over shorter and longer time scales as the analyses revealed both population-level plasticity and individual-level variation. These findings contrast general predictions of a global wide extinction risk of lizards as a result of climate change, and thus highlight the importance of accounting for evolutionary potential in behaviours in predictions of climate change effects on species and populations.