The 'Things You Do' encompass five types of actions that are strongly associated with good mental health: Healthy Thinking, Meaningful Activities, Goals and Plans, Healthy Habits, and Social Connections. Ultra-brief interventions which increase how often people perform these actions may decrease depression and anxiety. A two-arm randomized controlled trial (N = 349) compared an unguided ultra-brief intervention based on the 'Things You Do' against a waitlist control. The intervention included one online module, two practice guides, and four weeks of daily text messages. The primary timepoint was 5-weeks post-baseline. The intervention resulted in moderate reductions in depression (d = 0.51) and anxiety (d = 0.55) alongside moderate increases in the frequency of Things You Do actions (d = 0.54), compared to controls. No significant change in number of days out of role or life satisfaction were observed. Treatment completion was high (92 %), most participants reported being satisfied with the treatment (66 %), and improvements were maintained at 3-month follow-up. This study demonstrated that an automated ultra-brief 'Things You Do' intervention resulted in clinically significant reductions in depression and anxiety. Ultra-brief interventions may provide a scalable solution to support individuals who are unlikely to engage in longer forms of psychological treatment.