Expression facilitates the externalization of personal experiences and inner states (e.g., thoughts and emotions) embedded in everyday life. Yet, in mental health contexts, expression is often marginalized or systematically and structurally excluded from technology and system design. While HCI scholarship has explored expression in specific settings to advance user experience, a comprehensive understanding of expression remains limited. This limitation constrains the design space for future technologies that support and embrace expression, resulting in a potential lack of authentic experiential data reflecting individuals’ lived experiences embedded in everyday management of mental health. Through a systematic review of 𝑛 = 105 studies, we explore the mediums, populations, and practices of expression in mental health contexts. Our study contributes a nuanced sociotechnical understanding of expression for the HCI community, developing two concepts for enriching expression and offering insights for designing experience-rich technologies that better support expression for everyday mental health management.