The purpose of this experiment was to test the effects of a $500 per month guaranteed income for 2 years on health and financial outcomes. A mixed-methods randomized controlled trial in Stockton, CA, USA enrolled 131 individuals to the treatment condition and 200 to control to receive a guaranteed income from February 2019 to January 2021. Quantitative data collection began 3 months prior to allocation at 6-month intervals concluding 6 months after withdrawal of the intervention. Qualitative data collection included 105 interviews across 3 stages. The primary outcomes were income volatility, physical and mental health, agency, and financial wellbeing. The treatment condition reported lower rates of income volatility than control, lower mental distress, better energy and physical functioning, greater agency to explore new opportunities related to employment and caregiving, and better ability to weather pandemic–related financial volatility. Thus, this study provides causal evidence of positive health and financial outcomes for recipients of guaranteed income. As income volatility is related to poor health outcomes, provision of a guaranteed income is a potentially powerful public health intervention.