Purpose To explore the results of using the mini-ring Ilizarov external fixator for thumb metacarpal lengthening and its compatibility with a simultaneous groin flap. Methods From May 2016 to June 2019, 17 adult patients with thumb loss were treated with metacarpal lengthening using a mini-ring Ilizarov device. The device was composed of 2 rings, threaded rods, nuts, and K-wires (diameter, 1.5 mm). Of these patients, 6 also underwent simultaneous groin flap transfer. Lengthening was started 3 days after surgery at a rate of 0.66 mm/d. The pedicle of the groin flap was divided 1 month after the surgery. The healing index (days per cm), which denotes the number of days the external fixator is attached to the bone per centimeter of length gained, was used to evaluate the lengthening efficiency. Results The patients were observed for 21.9 + 9.0 months. The lengthening continued for 29.1 + 4.5 days, resulting in an additional length of 1.9 + 0.3 cm. Conclusions The mini-ring Ilizarov external fixator is a simple device for primary metacarpal lengthening. This device can be used with a groin flap for single-stage lengthening of injured thumbs with bone exposure. (J Hand Surg Am. 2024;49(7):708.e1-e6. Copyright (c) 2024 by the American Society for Surgery of the Hand. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-ncType of study/level of evidence Therapeutic IV.