The overall aim of this doctoral research was to study competitive and high performance endurance athletes' experiences and management of exercise-induced pain (EIP), mental performance (i.e., self-regulation, mindfulness), mental health, and mental illness symptoms. A mixed methods experimental design guided by the pragmatic stance was employed across three studies to (a) investigate how elite endurance athletes experience and manage EIP (Study 1), (b) compare the effectiveness of an online self-regulation and mindfulness intervention in improving EIP catastrophizing, mental performance (i.e., self-regulation, mindfulness), mental health, and mental illness symptoms in middle-distance runners (Study 2), and (c) qualitatively examine the impact of the online intervention focusing on self-regulation or mindfulness and designed to help endurance athletes improve EIP management, mental performance (i.e., self-regulation and mindfulness), and mental health (Study 3). Study 1. With a paucity of research investigating EIP management in elite endurance sports, the purpose of Study 1 was to examine how elite endurance athletes experience and manage EIP, using an applied self-regulation lens to help inform the work of Mental Performance Consultants (MPCs). Individual semi-structured interviews were conducted with 12 female and 3 male athletes (Mage = 23.73, SD = 2.31) competing in track and field (i.e., 600-1500 m; n = 5), swimming (i.e., 200-400 m; n = 5), and canoe kayak (i.e., 500-1000 m; n = 5). Given the centrality of self-regulation and the necessity to effectively manage internal states (e.g., EIP) in elite sport, the social cognitive model of self-regulation was employed to guide Study 1. The codebook thematic analysis generated two themes and six subthemes (i.e., sensations [burning, tightness, heaviness], beliefs [detrimental, mental, progressive]) related to the experience of EIP as well as three themes and 17 subthemes (i.e., preparation [accept and commit to EIP, recall sources of self-efficacy, develop a segmented performance plan, be accountable to training partners or coach, expose yourself to EIP when training, expose yourself to EIP when warming up, use imagery, implement typical pre-performance routine; execution [direct attention away from EIP, use instructional/motivational self-talk, implement a segmented performance plan, regulate breathing and relax, accelerate pace, self-monitor]; evaluation [reflect using a training journal, identify possible explanations, talk with a coach]) related to the management of EIP. Findings suggest that the experience of EIP is highly cognitive and generally perceived as detrimental to performance if not effectively managed. Athletes used several psychological strategies to prepare to experience EIP, reduce the aversive effects of EIP while performing, and learn from their EIP management strategies to improve their coping capacity. In terms of the number of reported psychological strategies, findings suggest that those used to prepare to experience EIP seem to be a priority. In general, the most popular strategies pertained to accepting and committing to experiencing EIP and directing attention away from EIP. Novel strategies not typically reported in the literature included exposing oneself to EIP when warming up, being accountable to training partners or coaches, using imagery to rehearse reactions to EIP, and accelerating one's pace. Importantly, combining self-regulation and mindfulness strategies appears to be key to effectively manage EIP. Study 2. No research has compared the impact of online sport psychology interventions on various outcomes affecting endurance athletes while employing an active control group. The purpose of Study 2 was therefore to compare the effectiveness of an online self-regulation and mindfulness intervention in improving EIP catastrophizing, mental performance (i.e., self-regulation, mindfulness), mental health, and mental illness symptoms in middle-distance runners. Using a pre-post experimental design, 61 middle-distance runners competing at provincial to international levels were allocated to either a self-regulation, mindfulness, or active control group using stratified randomization. They completed a pre-intervention survey measuring EIP catastrophizing, mental performance (i.e., self-regulation capacity, dispositional mindfulness), mental health, and mental illness symptoms (i.e., anxiety, depression, and eating disorder). A final sample of 52 participants (i.e., 17 self-regulation, 19 mindfulness, and 16 active control) completed the interventions and a post-intervention survey measuring the same outcomes addressed in the pre-intervention survey. Contrary to hypotheses, results from multiple mixed ANOVAs indicated that while mean scores trended in the positive direction between Time 1 and Time 2, the self-regulation and mindfulness interventions did not significantly differ from the active control intervention on the targeted outcomes. Interestingly, when excluding athletes who screened positive for mental illness symptoms (i.e., anxiety, depression, and/or eating disorder), the active control intervention was more effective in reducing anxiety symptoms than the mindfulness intervention. Overall, the self-regulation and mindfulness interventions were not any more effective than the active control intervention in improving the selected outcomes. Study 3. The purpose of this study was to qualitatively examine the impact of the online sport psychology intervention from Study 2, which focused on either self-regulation or mindfulness and was designed to help endurance athletes improve EIP management, mental performance (i.e., self-regulation, mindfulness), and mental health. A sample of 16 middle-distance runners (i.e., 11 women and 5 men) aged between 18 to 25 years old (Mage = 21.31, SD = 2.18) who participated in the eight-module SI or MI were purposefully selected based on their high, moderate, and low pre-post evolution scores. The codebook thematic analysis generated three themes and 13 subthemes (i.e., EIP management [reframing, understanding, self-talk, segmented performance plan]; mental performance [attention regulation, motivation, self-monitoring, self-efficacy, acceptance, defusion]; mental health [self-compassion, autonomy, support]) related to the participants' perceived changes as well as three themes and seven subthemes (i.e., delivery format [asynchronous, synchronous], content [applicability, audio and video files, examples, metaphors], timing [alignment with competitive season]) related to participants' recommendations. Findings suggest that both the self-regulation and mindfulness intervention positively impacted EIP management, mental performance, and mental health. Self-regulation and mindfulness should therefore be seen as complementary rather than conflicting or incompatible approaches. Furthermore, athletes reported that their EIP literacy and EIP management skills were limited at the onset of the intervention. Consequently, an educational component should continue to be integrated in future interventions to provide an overview of EIP and relevant mental performance skills to manage it. Given the prevalence of stressors and mental health challenges in competitive and high performance sport, screening for both positive mental health and mental illness symptoms at the onset of interventions is recommended so that athletes partaking in applied sport psychology studies can obtain appropriate mental health care and support as needed. Lastly, a hybrid delivery format incorporating both asynchronous and synchronous options may be the most effective when providing online sport psychology interventions. Overall, the findings of the current doctoral research suggest that both self-regulation and mindfulness strategies positively impact EIP management, mental performance, and mental health in endurance athletes. This inquiry is also the first to quantitatively and qualitatively compare the effectiveness of an online asynchronous self-regulation and mindfulness intervention in improving key outcomes affecting endurance athletes. With increased applied research and mental performance consulting occurring virtually due to the COVID-19 pandemic, it is vital to continue examining the quality and impact of online interventions on athletes.