Health care lies at the forefront of the impacts of climate change. Since the health sector is a major polluting and emission intensive sector, it remains a crucial challenge to address sustainability. The English National Health System (NHS) aims to be the first in the world to achieve net zero in all emission classes (Scope 1-3). In Germany, sustainability in health care is being driven bottom-up, while the Federal Ministry of Health at the time of the research in early 2021 takes no active stance on a net zero health care system. This article analyses the approaches to sustainability in the two different health care systems, explores common challenges, and draws recommendations to support the transition of the sector to a net zero future. An exploratory mixed method approach was taken applying qualitative and quantitative methods. This includes high-level expert interviews and an online survey from the United Kingdom (UK) and Germany. Results reveal the complex nature of health care systems and the need for engraining a systems-thinking approach. The findings call for the legal embedding of sustainability into the key principles of health care in Germany, endorses the ambition of the national health care systems in the UK, recommends collaborative cross-sector approaches for sustainability, and highlights the need for increased public awareness on the interrelation between human and planetary health to enable governance for sustainable health care.