The present case study puts forth the first idiographic application of a one to one Rational Emotive Behavior Therapy (REBT; Ellis in J Individ Psychol 13:38–44, 1957) intervention integrated with Motivational Interviewing (MI; Miller and Rollnick in Motivational interviewing: helping people change, Guilford Press, London, 2013) with a client (elite athlete) on irrational beliefs, self-determined motivation, and sporting performance. Building and maintaining a strong working alliance with a client is considered central to both MI and REBT, and psychological interventions generally. Whilst there are widespread recognition and recommendations of the importance of working alliance, the guidance on how to explicitly cultivate this beyond broad descriptions is relatively scant. Using the present case of ‘Theo’, MI was used as an identifiable and measurable framework to foster a strong working alliance between the practitioner and client, to increase Theo’s readiness and enhance the effects of the REBT intervention. After receiving eight one-to-one REBT sessions, data indicated acute and maintained reductions in Theo’s endorsement of irrational beliefs, increases in self-determined motivation, and marked increases in sporting performance. In addition, measures of treatment fidelity showed the effective and maintained inclusion of core MI principles through the REBT intervention. Ultimately, the case supports the effective application of REBT in addition to MI, whilst providing guidelines by which practitioners can actively facilitate a working alliance when applying REBT. Further, the case contributes to the emerging literature that links reductions in irrational beliefs with enhanced self-determined motivation and its potential value on wellbeing and/or performance.