COVID-19 affected the well-being of healthcare workers (HCWs) globally. Mental health app interventions (MHAIs) may offer appropriate and accessible means to support HCWs’ mental health. We conducted a pilot randomised controlled crossover trial involving 34 clinicians randomised to either a MHAI or a waitlisted group. After one month, outcome assessments were repeated and the waitlisted group then crossed over to the MHAI; they again completed outcome assessments after a month. The primary outcomes were feasibility, assessed with the Systems Usability Scale (SUS), and acceptability, assessed with the Client Satisfaction Questionnaire (CSQ). Secondary outcomes included efficacy for various mental health parameters. The SUS and CSQ scores indicated above average feasibility and acceptability. There was a significant difference in anxiety from baseline to 1-month follow-up between the groups, with greater improvement in the MHAI group. The groups differed in resilience and patient-related burnout from baseline to 1-month follow-up, with a trend towards significance, with greater improvements in the MHAI group. Anxiety and acute stress disorder severity improved significantly from pre- to post-intervention. We demonstrated that MHAIs hold potential for improving well-being of HCWs, although these findings will need to be replicated in adequately powered trials.