Electric-field oscillations are now experimentally accessible in the THz-to-PHz frequency range. Their measurement delivers the most comprehensive information content attainable by optical spectroscopy – if performed with high sensitivity. Yet, the trade-off between bandwidth and efficiency associated with the nonlinear mixing necessary for field sampling has so far strongly restricted sensitivity in applications such as field-resolved spectroscopy of molecular vibrations. Here, we demonstrate electric-field sampling of octave-spanning mid-infrared waves in the 18-to-39 THz (600-to-1300 cm−1 ) spectral region, with amplitudes ranging from the MV/cm level down to a few mV/cm. We show that employing powerful 2-µm gate pulses is key to approaching the ultimate detection limit of capturing all photons in the temporal gate, as well as providing high linearity with respect to the detected mid-infrared field. This combination of detection sensitivity, dynamic range, and linearity enables the exploitation of the full potential of emerging high-power waveform-controlled infrared sources for (non-)linear spectroscopy of solids, liquids, and gases.