A case report describing two men (aged 58 and 66 years) with chronic thumb metacarpophalangeal (MCP) radial collateral ligament (RCL) injuries demonstrated that reconstruction using a modified abductor pollicis brevis (APB) tendon transfer restored joint stability and enabled return to daily activities. Both patients presented with pain, weakness, and instability, with imaging showing volar subluxation of the proximal phalanx. Surgical reconstruction using a dorsal half-slip of the APB tendon, combined with temporary K-wire stabilization and postoperative immobilization, resulted in satisfactory alignment and function at 3 months, with no complications or radiographic failure at final follow-up (2 years and 6 months, respectively). A 20° reduction in MCP flexion was observed, but patients returned to normal activities without recurrent instability.