A method of implanting a lateral unicondylar knee prosthesis in a lateral articulating portion of a femur having a lateral condyle for engagement with a tibia, the method including the steps of: bending a patients knee such that the knee is not in full extension; making an incision through the skin, muscle, and other soft tissue until the damaged bone surfaces are exposed; resecting an end portion of the lateral tibia; resecting an end portion of the lateral femoral condyle; attaching a tibial prosthetic component to the resected end portion of the tibia; attaching a lateral femoral condyle prosthetic component to the resected end portion of the lateral femoral condyle; determining the thickness of a mobile bearing member with the knee in full extension; and inserting the mobile bearing member between the tibial prosthetic component and the femoral prosthetic component.