Digital mental health interventions comprise a potentially effective and accessible form of support for young people, particularly at times when traditional face-to-face service delivery is reduced, as in the COVID-19 pandemic. This study assessed the demographic profile of young people using a digital mental health support service and evaluated outcome change over the course of a structured online counselling intervention (synchronous text-chat sessions with a practitioner). The data were collected from 23,260 young people aged between 10 and 25 years engaging with the intervention between April 2019 and June 2021. Young people accessing these services had high levels of mental health needs, particularly those identifying with non-binary gender identity. Service users were mostly female, with equitable rates of access for young people from racialised communities. Overall outcome change demonstrated small effect sizes according to the YP-CORE (0.19) and CORE-10 (0.38), which increased to a moderate level when young people remained engaged with a dedicated practitioner for at least seven sessions (0.38, 0.58). Regression analysis illustrated the effect of the number of sessions on outcome change, but this can be also influenced by other variables such as age and gender. Further research is required to explore ways to engage with young people using digital web-based services for a longer period and to collect and analyse single-session outcome data.