This dissertation investigated the role of mental habits in well-being. There has been extensive research on how people form behavioural habits related to their physical health (e.g., diet and exercise), but there have been fewer theoretical and empirical attempts to understand how the same habitual processes might underlie mental health. By better understanding the role of habits in mental health, particularly habitual thinking, clinicians and researchers might be able to improve the treatments offered for mental health concerns (e.g., depression and anxiety). In the three studies presented in this dissertation, I aimed to integrate habit research with the study of mental health by examining the extent to which thoughts can be considered habitual. In Study 1, we conducted a scoping review to describe and summarize the existing literature on mental habits (as they relate to mental health). We screened 2817 articles and included 20 in the review. Of these 20 articles, we examined 24 separate studies and 4 commentaries on mental habits. When defining habits, researchers emphasized the importance of automaticity (described in 80% of articles) as a key factor in differentiating mental habits from other thinking processes. Most research studies used correlational research designs (71%) with university student samples (75%) measuring various constructs including negative self-thinking, worry, self-critical thinking, self-stigma, negative body image thinking, and emotion regulation. We found no articles that measured positive mental habits. Lastly, the concept of mental habits has been poorly integrated with related psychological constructs (e.g., automatic thinking, repetitive negative thinking). In Study 2, we developed two new measures of mental habits, the Positive Thought Automaticity Index (PTAI) and Negative Thought Automaticity Index (NTAI), that integrated the strengths of existing mental habit and automatic thought questionnaires. We then used these two measures to tease apart the roles of automaticity and frequency in predicting well-being outcomes. Based on two samples of participants from the United Kingdom and Canada, these new measures demonstrated predictive and concurrent validity, internal consistency reliability, and test-retest reliability. Positive and negative thought automaticity predicted well-being outcomes over and above thought frequency. Thought automaticity partially mediated the relationship between thought frequency and how much participants believe thoughts to be true. Overall, the results of Study 2 provided evidence of the utility of automaticity as a distinct thinking process compared to frequency. In Study 3, we examined a key component of mental habits that has been under-explored in the literature: the cues that precede automatic thoughts. Participants from the United Kingdom completed a questionnaire about the recent and past cues that have preceded their automatic thoughts. We analyzed the results using both qualitative and quantitative methods. Using thematic analysis, we coded participants responses using the PERMA (Positive Emotion, Engagement, Relationships, Meaning, and Achievement) framework. We coded 92% of situations preceding negative thoughts and 97% of situations preceding positive thoughts as fitting within the PERMA framework. Participants described multiple cues preceding the same thought, with a median of five cues reported for both positive and negative thoughts. Participants also endorsed experiencing internal (e.g., emotions) and external (e.g., situations) cues as preceding thoughts in similar proportions. The results from Study 3 contribute to the mental habit literature by providing important information about the nature of the events preceding automatic thinking. Overall, this program of research connects several disparate areas of study (e.g., mental habits, automatic thinking) and provides new insights about how the conceptual framework of mental habits may be useful to both clinicians and researchers. By summarizing the existing research on mental habits (Study 1), distinguishing automatic from frequent thinking (Study 2), and better understanding the cues that precede automatic thoughts (Study 3), this dissertation attempts to clarify the role of habitual thinking in mental health. My intention is that the ideas in this dissertation will stimulate further discussions amongst habit researchers and clinicians, which will improve our understanding of mental health problems and how we treat them.