The biomechanical properties of the rabbit medial collateral ligament (MCL) as a function of maturation and age were investigated. Femur–MCL–tibia (F‐M‐T) preparations were obtained from rabbits of different age groups (open or closed epiphysis). Parallel increases in the animal body weight and ligament cross‐sectional area were recorded with age. Cyclic and tensile failure tests were performed to obtain the structural properties of the F‐M‐T complex and the mechanical properties of the MCL substance. There were significant increases in the load at failure, energy‐absorbing capability of the bone–ligament junction, and in the tensile strength of the ligament substance as a result of maturation and subsequent aging. Increases in the area of hysteresis obtained during cyclic loading–unloading were also documented. At the closing of the epiphysis, the mode of failure of the F‐M‐T structure progressed from tibial avulsion to failure in the midsubstance of the ligament. An asynchronous rate of maturation was observed between the structural properties of the bone–ligament complex and the mechanical properties of the ligament substance.