This study aims to compare the clinical, radiological, and functional outcomes of the late-presenting ulnar carpometacarpal (CMC) joint injuries treated conservatively with plaster cast versus treated surgically with open reduction internal fixation (ORIF). Between May 2019 - October 2021, a total of 28 patients (26 males, 2 females; mean age: 32.2±10.3 years; range, 20 to 59 years) who were treated conservatively or surgically were retrospectively analyzed. Fourteen patients operated with ORIF were included in the first group (surgery group), and 14 patients followed conservatively with a plaster cast were included in the second group (conservative group). The patients were classified according to Cain's classification and the AO Foundation and Orthopedic Trauma Association (AO/OTA) classification. The patients were evaluated in terms of pulp palm distance (PPD), Visual Analog Scale (VAS), Disabilities of the Arm, Shoulder and Hand (DASH) score, grip strength, time to return to work, follow-up time, and presence of complications. No significant difference was found in terms of the pain and functional scores. In the conservative group, the grip strength of the injured side was significantly lower than the healthy side (p=0.0093). The patients with and without metacarpal fracture subluxation/dislocation were evaluated separately, and the grip strength of the fractured side in the subluxation/dislocation group was found to be significantly higher in the surgery group than the conservative group (p=0.0237). In the group with subluxation/dislocation, the recovery time increased, as the time to treatment increased. In three patients in the conservative group, the PPD values were 2, 3, and 4 mm, respectively while it was 0 mm for all in the surgery group. The non-bridging dorsal buttress plate technique with or without a Kirschner wire is effective in patients with delayed ulnar CMC fracture-dislocations. Although surgery is associated with longer time to return to work, long-term results obtained with anatomical reduction of the joint are satisfactory for manual workers.