Background Functional Neurological Disorder (FND) encompasses motor, cognitive, and sensory symptoms resulting from disruptions in brain-body communication. Emerging research suggests a higher-than-expected occurrence of autism in FND, potentially due to shared cognitive mechanisms and overlapping comorbidities. However, large-scale characterisation of this dual-diagnosis is lacking. Methods Using de-identified health records from the TriNetX research network, we identified children and adults with both FND and autism (‘FND + Autism’), comparing them to individuals with FND only (‘FND-only’) and autism only (‘Autism-only’). We examined psychiatric comorbidities (e.g. mood, anxiety, post-traumatic stress disorder, personality disorders, obsessive–compulsive disorder), intellectual disability and ADHD. Results Of 220,312 individuals with an FND diagnosis, and 674,971 individuals with an autism diagnosis, 5,152 (2.3% of FND, 0.76% of autism) had both FND and autism. The rates of autism were therefore 6 times higher in FND compared to the base rates of the TriNetX population. Most were diagnosed with autism before FND, with over one-third diagnosed in childhood. Functional seizures were the most common FND subtype, and were more frequent in FND + Autism than FND-only (adults: 52% vs. 44%; children: 47% vs. 42%). Comorbidity across all psychiatric conditions was significantly higher in FND + Autism compared to both comparison groups. ADHD was particularly elevated in FND + Autism (adults: 50% vs. 13% FND-only, 36% Autism-only; children: 64% vs. 21% FND-only, 41% Autism-only). Conclusions This study presents the largest dataset to date characterising individuals with co-occurring FND and autism. Findings are consistent with previous findings of higher rates of autism in people with FND and reveal a potentially distinct clinical profile, marked by elevated rates of ADHD and psychiatric comorbidities, and increased occurrence of functional seizures compared to FND- or Autism-only groups. Recognising this overlap may improve diagnosis, clinical care, and understanding of mechanisms underlying the co-occurrence of FND and autism.