This study evaluated biomechanical properties of healing ligament following subfailure (grade II) injury by comparing young and mature animals in a rat lateral collateral ligament (LCL) model. One randomly selected LCL was stretched in situ using a custom designed device in eighteen young (21 days) and eighteen skeletally mature (8 months) male rats. Animals were euthanized at 0, 7, and 14 days post-surgery, and ligament ultimate stress, strain at failure and laxity were determined (n = 6 pairs per group). At time 0 after introduction of stretch injury, ligament laxity was present in both groups. The mature rats had 54 ± 9% strength of the control while the immature rats had 58 ± 11% of the strength of the control, representing a consistent and significant injury. The immature and mature ligaments showed similar patterns of cellular damage post-injury and had similar modes of mechanical failure. Ligament laxity decreased in each group as healing time increased, however ligament laxity did not completely recover in either group after 2 weeks of healing. After 7 and 14 days of healing, the mature rats, respectively, had only 63 ± 14% and 80 ± 8% strengths of the controls while the immature rats had 94 ± 6% and 94 ± 10%. Hence, mechanical data showed that immature animals recovered their strength after a grade II sprain at a faster rate than mature animals. However, ligament laxity was still present in both groups two weeks after the injury and was not completely removed by growth in the immature group. These findings are clinically relevant since joint laxity after injury is common, and these results may explain the presence of continued instability in a joint injured at a young age. Hence, this study, with a new injury model, showed differences in ligament healing associated with maturity and quantified the clinically observed persistance of ligament laxity.