Ultrathin metasurfaces with record-high nonlinear optical response of 1.2 × 106 pm V−1 for second harmonic generation are experimentally demonstrated in the mid-infrared spectral range. A second harmonic power conversion efficiency of 0.075% is achieved in a 400-nm-thick (λ/25) metasurface at a pump intensity of only 15 kW cm−2.