Rural poverty and lack of access to education has led to urban migration and fed the constant growth of urban slums in Lima, Peru. Inhabitants of these informal settlements lack land rights and access to a public water supply, resulting in poor sanitation, an inability to grow food, and suboptimal health outcomes. A repeated measures longitudinal pilot study utilizing participatory design methods was conducted in Lima between September 2013 and September 2014 to determine the feasibility of implementing household gardens and the subsequent impact of increased green space on well-being. Anthropometric data and a composite of five validated mental health surveys were collected at the baseline, 6-months, and 12-months after garden construction. Significant increases from the baseline in all domains of quality of life, including: physical (p < 0.01), psychological (p = 0.05), social (p = 0.02), environmental (p = 0.02), and overall social capital (p < 0.01) were identified 12 months after garden construction. Life-threatening experiences decreased significantly compared to the baseline (p = 0.02). There were no significant changes in parent or partner empathy (p = 0.21), BMI (p = 0.95), waist circumference (p = 0.18), or blood pressure (p = 0.66) at 6 or 12 months. Improved access to green space in the form of a household garden can significantly improve mental health in an urban slum setting.