We report the soft-aperture Kerr-lens mode-locked Cr:ZnS laser, generating 550 mW of 69 fs nearly transform-limited pulses at 2.39 μm wavelength. The pulse energy reached 3.8 nJ at 145 MHz repetition rate, limited by the onset of double-pulsing. This corresponds to the shortest-pulse and highest-energy direct femtosecond laser source in the mid-infrared. Dispersion compensation was achieved by a single chirped mirror and a thin sapphire plate, making the laser design simple, compact and very stable, and operating at ambient air and room temperature. The superb thermal and mechanical properties of Cr:ZnS, exceeding those of Cr:ZnSe and many established femtosecond laser crystals, should allow for further scaling of output power.