Background The relationships between positive and negative emotional experience and physical and psychological well-being have been well-documented. The present study examines the prospective positive relationship between concurrent positive and negative emotional experience and psychological well-being in the context of psychotherapy. Methods 47 adults undergoing psychotherapy completed measures of psychological well-being and wrote private narratives that were coded by trained raters for emotional content. Results The specific concurrent experience of happiness and sadness was associated with improvements in psychological well-being above and beyond the impact of the passage of time, personality traits, or the independent effects of happiness and sadness. Changes in mixed emotional experience preceded improvements in well-being. Conclusions Experiencing happiness alongside sadness in psychotherapy may be a harbinger of improvement in psychological well-being.