Nanoelectronic devices play an essential role in many domains, and their development and improvement attract considerable attention in fundamental and applied research. Access to the local physical processes involved in these nanosystems during their operation is, therefore, crucial. We show how electric fields in real nanodevices can be studied under working conditions using operando electron holography. A specific sample preparation method was first developed to bias electron-transparent nanodevices extracted from production lines while ensuring their electrical connectivity and functionality without employing dedicated probe-based holders. Metal–insulator–metal nanocapacitors were prepared using this approach based on focused ion beam circuit modification. Operando electron holography allowed the electric potential to be quantitatively mapped in the active areas, and between devices, while biasing the devices in situ. Experimental results were compared with finite element method modeling simulations to determine local electrical parameters. We demonstrate that electrical properties, such as capacitance and surface charge density, can be measured at the nanoscale and have been preserved by our sample preparation methodology when compared to macroscopic measurements. This work paves the way for mapping the local electrical properties of more complex biased devices.