Adolescence and early adulthood (ages 16- to 24-years) are formative years when young people begin to engage in sexual behaviours and relationships. Little is understood about how mental health impacts risky sexual behaviour in this age group. Impulsivity is often thought to be a mediating factor in risky sexual behaviour in adult populations, and impulsivity is suggested to be greater in this young people. This thesis aimed to address this by conducting a two-part project. The first empirical paper addresses the question whether there are any differences in terms of risky sexual behaviour and impulsivity between young people with a diagnosis of borderline personality disorder, young people with any other mental health diagnosis, and young people without an existing mental health diagnosis. Findings from this study indicate that there are no significant differences in impulsivity and risky sexual behaviour between young people with mental health diagnoses, including BPD, and those without. Instead, results showed that impulsivity was the primary indicator for risky sexual behaviour in this age group. The second empirical paper seeks to answer the question how young people with a diagnosis of borderline personality disorder make sense of their lived experiences of sexual relationships. Findings from this second study indicated that emotional dysregulation experienced by the participant led to them seeking comfort by using drugs and alcohol and engaging in sexual behaviours. A second key point from this second study was that heterosexual sex and relationships were a way for the young participant, who identified as non-binary and gay, to fit feel like they fit into a heteronormative society. Implications for future research and clinical practice are discussed.