We identify and experimentally demonstrate a physical scenario whereby high-peak-power mid-infrared (mid-IR) pulses can be compressed as a part of their free-beam spatiotemporal evolution within the regions of anomalous dispersion in air to yield few-cycle subterawatt field waveforms. Unlike filamentation-assisted pulse compression, the pulse-compression scenario identified in this work does not involve any noticeable ionization of air, enabling a whole-beam self-compression of mid-IR laser pulses without ionization-induced loss. Ultrashort high-peak-power 3.9 μm laser pulses are shown to exhibit such self-compression dynamics when exposed to the dispersion anomaly of air induced by the asymmetric-stretch rovibrational band of carbon dioxide. Even though the group-velocity dispersion cannot be even defined as a single constant for the entire bandwidth of mid-IR laser pulses used in experiments, with all soliton transients shattered by high-order dispersion, 100–200 GW, 100 fs, 3.9 μm laser pulses can be compressed in this regime to 35 fs subterawatt field waveforms.