Studies have shown that there are differences between the presentations of males and females diagnosed with autism. There remains a developing understanding about how the presentation of autism differs between boys (hereafter referred to as 'assigned males at birth') and girls (assigned females at birth). This study sought to investigate the presence of sex differences in autistic children. Participants (1.11-17.97 years) attended an assessment clinic and participated in measures of intelligence/development, social/communication skills and behaviour. Adaptive skills were evaluated using a range of standardised measures, and other clinical and demographic variables were collected (e.g. age, intelligence quotient, ratio of male to female). Assigned males at birth displayed more autism characteristics and greater symptom autism severity than assigned females at birth. No significant differences were found between assigned males at birth and assigned females at birth on any measure of intelligence. Children assigned males at birth received assessments 6 months earlier than children assigned females at birth on average. Externalising behaviour problems were more evident in assigned males at birth, but statistically significant differences in adaptive skills were not apparent between assigned males at birth and assigned females at birth. This study showed assigned females at birth and assigned males at birth differ in autism symptoms and severity and age at diagnosis based on a real-world sample. It highlights the importance of balancing assessments of symptoms with assessment of adaptive function.