Objectives: Strong cervical musculature is recognized as a protective factor against sports-related concussions. Evidence suggests that jaw clenching may activate cervical muscles, potentially reducing head acceleration during impact. Methods: This observational cohort study examined the immediate effects of a customized interocclusal orthotic (CIO) on cervical muscle performance. Forty-two healthy adults (≥18 years) underwent strength and endurance testing with and without a CIO using a digital pressure gauge and six directional isometric contractions. Descriptive statistics and two-way repeated-measures MANOVA models were applied to evaluate condition effects. Results: CIO use produced significant improvements in cervical muscle strength and endurance across all directions compared to non-use. Forward flexion strength increased by 12.96% (p < 0.001, ηp² = 0.185), backward extension by 10.34% (p = 0.017, ηp² = 0.091), right rotation by 19.03% (p < 0.001, ηp² = 0.333) and left rotation by 19.86% (p < 0.001, ηp² = 0.353). Endurance gains demonstrated large effect sizes, with flexor endurance improving by 44.57% (p < 0.001, ηp² = 0.447). Conclusions: Optimized jaw alignment using a customized orthotic can elicit immediate, clinically meaningful enhancements in cervical strength and endurance, suggesting a promising adjunct for concussion risk mitigation in contact sports.