Whereas reminiscence‐based interventions that focus on reducing depression and pessimism have been shown to be effective, most studies have employed longer term treatments. There has also been a tendency for reminiscence‐based studies to focus on the benefits of reminiscence for older adults, with few studies involving younger adults. This study examined the efficacy of a one‐session reminiscence‐based intervention for reducing depressed affect and pessimism in younger adults. A total of 26 non‐clinical participants were administered a one‐to‐one guided interview in which they were encouraged to recall past problem‐solving successes, and the lessons learnt from these successes. Measures of affect, pessimism, and mastery were administered pre‐ and post‐interview. Levels of depressed affect, mastery, and pessimism significantly improved following the interview, compared with baseline levels. Changes in mastery were associated with changes in pessimism, but not with changes in depressed affect. This study suggests that a one‐session reminiscence‐based intervention may significantly contribute to the improvements in depressed affect, pessimism, and mastery in a younger cohort. It also suggests that the improvement in pessimism may be accounted for by improvement in self‐mastery.