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Achieving high crystalline quality germanium‐tin (Ge1 − xSnx) semiconductors at Sn content exceeding 10% is quintessential to implement the long sought‐after silicon‐compatible mid‐infrared photonics. Herein, by using sub‐20 nm Ge nanowires as compliant growth substrates, Ge1 − xSnx alloys with a Sn content of 18% exhibiting a high composition uniformity and crystallinity along a few micrometers in the nanowire axial direction are demonstrated. The measured bandgap energy of the Ge/Ge0.82Sn0.18 core/shell nanowires is 0.322 eV enabling the mid‐infrared photodetection with a cutoff wavelength of 3.9 µm. These narrow bandgap nanowires are also integrated into top‐gated field‐effect transistors and phototransistors. Depending on the gate design, these transistors are found to exhibit either ambipolar or unipolar behavior with a subthreshold swing as low as 228 mV/decade at 85 K. Moreover, varying the top gate voltage from ‐1 to 5 V yields nearly one order of magnitude increase in the photocurrent of the nanowire phototransistor under a 2330 nm illumination. This study shows that the core/shell nanowire architecture with a very thin core not only mitigates the challenges associated with strain build‐up observed in thin films but also provides a promising platform for all‐group IV mid‐infrared photonics and nanoelectronics paving the way toward sensing and imaging applications. [ABSTRACT FROM AUTHOR]