Collagen fiber re-alignment is one postulated mechanism of tendon structural response to load. While collagen fiber distribution has been shown to vary by tendon location in the supraspinatus tendon (SST), changes in local re-alignment behavior have not been examined throughout postnatal development. Postnatal tendons, with immature collagen fibrils, may respond to load in a much different manner than collagen fibers with mature fiber-fiber and fiber-matrix connections. Local collagen fiber re-alignment is quantified throughout tensile mechanical testing in a developmental mouse SST model and corresponding mechanical properties measured. Collagen fiber re-alignment occurred during preconditioning for 28 day old tendons, at the toe-region for 10 day tendons and at the linear-region for 4 day tendon midsubstance. Mechanical properties increased with developmental age. Linear modulus was lower at the insertion site compared to the midsubstance location at all time points. Local differences in collagen fiber distributions were found at 10 and 28 days for all mechanical testing points (except the 10 day transition point). This study found that collagen fiber re-alignment depends on developmental age and suggests that collagen fibrillogenesis may influence the tendon's ability to structurally respond to load. Additionally, results indicate that the insertion site and tendon midsubstance locations develop differently.