The aim of this study was to assess the effect of (preventive) rehabilitation on swallowing and mouth opening after concomitant chemoradiotherapy (CCRT). Forty-nine patients with advanced oral cavity, oropharynx, hypopharynx and larynx, or nasopharynx cancer treated with CCRT were randomized into a standard (S) or an experimental (E) preventive rehabilitation arm. Structured multidimensional assessment (i.e., videofluoroscopy, mouth-opening measurement, structured questionnaires) was performed before and 10 weeks after CCRT. In both S and E arms, feasibility was good (all patients could execute the exercises within a week) and compliance was satisfactory (mean days practiced per week was 4). Nevertheless, mouth opening, oral intake, and weight decreased significantly. Compared to similar CCRT studies at our institute, however, fewer patients were still tube-dependent after CCRT. Furthermore, some functional outcomes seemed less affected than those of studies in the literature that did not incorporate rehabilitation exercises. Patients in the E arm practiced significantly fewer days in total and per week, but they obtained results comparable to the S arm patients. Preventive rehabilitation (regardless of the approach, i.e., experimental or standard) in head and neck cancer patients, despite advanced stage and burdensome treatment, is feasible, and compared with historical controls, it seems helpful in reducing the extent and/or severity of various functional short-term effects of CCRT.