Current human meat consumption levels contribute to environmental degradation and are a risk factor for non-communicable diseases. Globally, meat-reduction policy interventions are limited. Meat-Free Mondays (MFMs) is a global campaign to reduce meat consumption to improve planetary and human health. We conducted a mixed methods evaluation of MFMs at three District Health Boards (DHBs) (one not considering a MFM policy, one that had trialled MFMs and one implementing MFMs) to investigate attitudes towards MFMs and barriers and enablers to implementation. An online staff survey and eleven semi-structured interviews with food service managers, café managers and sustainability managers were conducted. Of the 194 survey participants, 51% were actively cutting back on meat, mainly for health, environmental concerns and enjoyment of plant-based dishes, and 59% were positive towards MFMs. Qualitative analysis using a general inductive approach identified four themes: (1) ‘Change and choice’ (impact on personal choice), (2) ‘Getting it right’ (product and price, food quality, health, customer retention and sales), (3) ‘Human and planetary health’ (hospitals as leaders in healthy, sustainable diets), (4) ‘Implementation success’ (communication and education). Recommendations for implementation of MFMs included seeking feedback from other DHBs, wide consultation with food service staff, cultural and dietitian food service support and providing evidence of the success of MFMs and alternatives to MFMs.