Environmental conditions experienced by animals constrain their energy acquisition and its subsequent allocation to growth and reproduction, which ultimately contributes to population dynamics. Understanding how environmental conditions affect these physiological processes is therefore important for predicting how threatened species will respond to altered food and temperature conditions. Here we use a mechanistic modelling approach based on Dynamic Energy Budget (DEB) theory to demonstrate that changing food availability has a strong impact on growth and reproduction for a Western Australian population of green sea turtles (Chelonia mydas), particularly in scenarios with simulated marine heatwaves. Models predicted increasing time between nesting years in scenarios of decreasing food availability. Furthermore, increased frequency of marine heatwaves reduced reproductive output with the number of eggs a female produced in its lifetime predicted to be approximately 20% lower when heatwaves occurred every five years compared to every 20 years. Our predictions suggest that frequent marine heatwaves could have similar adverse effects to long-term decreases in food availability. In all scenarios, direct impacts of changes in temperature were less pronounced and suggest that the strongest impacts of the increasing temperatures of climate change will be mediated through food availability. The approach demonstrated here provides a strong foundation for understanding how the Ningaloo C. mydas population will respond to climate change, and can be refined as new physiological, behavioural, and environmental data become available.