Research exploring the connection between pet ownership and mental health has expanded substantially in recent years, yet scientific evidence remains inconclusive. Existing studies have oversimplified this relationship by focusing primarily on pet ownership itself, without accounting for crucial factors such as species of the pet, or important relationship dynamics such as owner-pet attachment orientations. This study sought to investigate whether the relationship between owner-pet attachment and owner mental health could be better understood through the lens of owner-perceived pet compatibility, perceived pet welfare, and pet behavioral issues. These aspects, often overlooked in previous research, are believed to play crucial roles in shaping owner-pet relationships and owner mental wellbeing. This study surveyed emerging adults (ages 18-26) from the UK (N=600) with anxiety and/or low mood who owned dogs and/or cats. A large portion of the sample met clinical criteria for Generalized Anxiety Disorder or Major Depressive Disorder. Our findings revealed that dog owners exhibited more secure pet attachments than cat owners. Attachment notably influenced mental health whereby anxious attachment was linked to poorer mental health among dog owners, while avoidant attachment was associated with better mental health in both dog and cat owners. Insecure attachment related to poorer pet quality of life, increased reports of pet behavioral problems, and poorer owner-pet compatibility, regardless of pet species. Additionally, poorer welfare and more behavioural problems were associated with poorer mental health for dog owners; these findings were not replicated for cat owners. Notably, a dog’s mental state (such as appearing depressed), as well as fear and anxiety in dogs, mediated the relationship between owner-pet attachment and owner mental health. Owner-dog compatibility, particularly in the affection domain, influenced owner anxiety, positively mediating the relationship between anxious attachment and poorer mental health, while negatively mediating the relationship between avoidant attachment and better mental health. These findings suggest that a simplistic view of pet ownership fails to capture the complexity of the factors that shape the mental health of pet owners and underscores the need to consider important owner-pet factors to fully understand how the human-pet relationship can impact the wellbeing of both people and their pets.