Importance: People with Duchenne muscular dystrophy (DMD) and spinal muscular atrophy (SMA) develop impaired oral function because of reduced temporomandibular joint range of motion (ROM), which affects feeding and oral hygiene activities of daily living (ADLs). Objective: To assess whether the TheraBite^®, an intraoral stretching device, improves ROM. Design: Case series, with intervention duration varying from 7 to 30 mo. Treatment frequency varied from weekly to consultative (several times per year). Setting: Varied depending on the ease of transportation for the participant and caregivers. Two participants were treated in an outpatient medical clinic. The other was provided consultative care during multidisciplinary medical clinics and completed a home program. Participants: Two adults with DMD and one with SMA. Intervention: Stretching protocol using the TheraBite. Outcomes and Measures: Temporomandibular active ROM (AROM) was determined using a disposable TheraBite oral goniometer. Passive ROM (PROM) was determined using the adhesive scale on the TheraBite. Measures were taken at baseline, each intervention or consultation, and the end of care. ADL participation and caregiver burden were measured at the end of intervention. Results: For participants with DMD, AROM remained unchanged, but PROM increased by 40%–65%. The participant with SMA demonstrated 33% and 47% improvements in AROM and PROM, respectively. Participants or caregivers reported improved feeding function, improved oral hygiene, or reduced fatigue. Conclusion: TheraBite may improve temporomandibular PROM in people with DMD and temporomandibular AROM and PROM in people with SMA. It may also improve ADL function and consequently reduce caregiver burden. Further investigation is warranted. What This Article Adds: Temporomandibular contracture in people with DMD and SMA contributes to reduced lifespan and loss of function. Use of the TheraBite with this population may preserve temporomandibular ROM and improve feeding, hygiene, and quality-of-life outcomes.