This study aims to measure the speech intelligibility of Mandarin-English speakers with dysarthria before and after phonation therapy, in order to determine the effectiveness of this approach. A within-group design was used with two case studies which allowed one to measure therapy variables (single word and sentences); language variables (Mandarin and English); and speech production variables (respiration, phonation, articulation, resonance, and prosody). Both participants demonstrated highly significant improvement in Mandarin intelligibility scores after therapy compared with minimal changes in English intelligibility. These results demonstrate for the first time that phonation therapy is effective in increasing intelligibility, for Mandarin more than for English. Phonation therapy is also effective in enhancing accurate tone production for all four tones of Mandarin. We discuss the evidence that phonation therapy is significantly more effective for rehabilitating Mandarin-English bilinguals with dysarthria in Mandarin (a tonal language) than in English (a non-tonal language).