As the race towards global Net Zero emissions continues, governments and businesses alike are increasingly being held accountable to sustainability pledges. Organisations are pressured to engage in environmentally friendly practices through policy, regulation, or market influences. Although many individuals and organisations recognise the importance of climate positive behaviour, they may struggle to consistently engage for various reasons. Often, these challenges arise due to organisations lacking structures or social orientations that prioritise environmental well-being, as many were established before climate concerns became as widespread as they are today. Consequently, individuals are often working against the organisational machine to switch to pro-environmental behaviours. Matching conventional wisdom: doing the right thing is rarely easy. To address this, we suggest that familiar business efficiency methodologies are reframed to incorporate and sustain pro-environmental behaviours within organisations. Specifically, we propose a ‘Sustainable-by-Design’ approach, drawing from established Lean principles to improve efficiency in an organisation by reducing waste. Waste reduction is often associated with a reduction in internal cost and embodied greenhouse gas emissions. The true Lean principles foster a more pleasant and less frustrated customer and employee experience, contrasting historic associations with unempathetic ‘efficiency improvements’ for maximising profits in manufacturing industry. Rather than solely focusing on output efficiency, true Lean principles focus on harmony (sustainability) of a system. The Sustainable-by-Design approach is based upon this ‘Lean, not mean’ ideal. We propose adding value by combining this approach with behavioural science principles, such that the Lean, climate-friendly behaviours become the ‘path of least resistance’. This is achieved by working closely with the organisation to identify their specific behavioural, cultural, psychological, and social barriers and enablers to the target behaviours. Empathetic engagement with the organisation’s people facilitates the embedding of concurrent structural and social changes, resulting in an optimised organisation that encourages sustained, sustainable behaviours at its core.