Low-grade waste heat is an abundant and underutilised energy source, and the promise of thermo-electrochemical cells to harvest this resource and power applications such as wearable devices and sensors is increasingly being realised. However, despite substantial advances in performance in recent years, understanding the interior processes occurring within these devices remains a challenge. Here we report an operando magnetic resonance imaging (MRI) approach that can provide quantitative spatial maps of electrolyte temperature and redox ion concentrations in functioning thermo-electrochemical cells. Time-resolved images are obtained from liquid and gel electrolytes, allowing the effects of redox reactions and competing mass transfer effects such as thermophoresis and diffusion to be visualised and correlated with the device performance via simultaneous electrochemical measurements. This method offers valuable insights into these devices and will greatly aid their future design and optimisation.