Epidermal electronics that can be mounted directly onto human skin to monitor biological signals have wide applicability, such as in healthcare and sports. This requires integrating various analogue and analogue/digital mixed-signal circuits on a flexible film as well as sensors. Flexible analogue circuits based on oxide semiconductors or amorphous Si have been realised, but so far they do not allow robust digital signal transmission to an external system, and they have a high operating voltage. Here, we report a low-voltage and fully flexible analogue/digital mixed-signal circuits based on carbon nanotubes (CNTs). The key contribution is a novel operational amplifier with a high gain and low operating voltage, which is important for negative-feedback circuits and dramatically suppresses the influence of device variability and instability. Fully flexible CNT-based mixed-signal circuits were realised monolithically on a plastic film for the first time and demonstrated stable and continuous operation at a low supply voltage.