Research suggests that a childhood diagnosis of Autism Spectrum Disorder (ASD) or Attention Deficit Hyperactivity Disorder (ADHD) is associated with poorer teacher-child relationship quality and higher rates of externalising behaviours. It is proposed that the socio-emotional problems experienced by those children with ASD and/or ADHD lead to poor teacher-child relationship quality and an increase in externalising behaviours. It is also proposed that self-regulation skills and language ability may mediate these relationships. Data from the Longitudinal Study of Australian Children (LSAC) from 5107 participants whose ages ranged from 2 to 3, 6 to 7 and 10 to 11 years were used for these analyses. Outcome variables were teacher-child relationship quality and externalising behaviours. There were direct effects between ADHD/ASD and externalising behaviours (SDQ). Externalising behaviours were also fully mediated by self-regulation. All other models showed significant partial mediation. Findings suggest that language was not significantly related to either outcome variable. These results support the conclusion that self-regulation is a protective factor for those diagnosed with ASD and other disorders with associated socio-emotional problems such as ADHD. Results also suggest that a focus on self-regulation skills in educational programs has the potential to improve socio-relational outcomes for all children with socio-emotional difficulties.