Successful social interactions are assumed to depend on theory of mind--the ability to represent others' mental states--yet most studies of the relation between theory of mind and social-interactive success rely on non-interactive tasks that do not adequately capture the "spontaneous" engagement of theory of mind, a crucial component of everyday social interactions. We addressed this gap by establishing a novel observational rating scale to measure the spontaneous use of theory of mind (or lack thereof) within naturalistic conversations ("conversational ToM"; cToM). In 50 age- and gender-matched dyads of autistic and typically developing youth aged 8-16 years (three dyad types: autistic--typically developing, typically developing--typically developing, autistic--autistic), we assessed cToM during 5-min unstructured conversations. We found that ratings on the cToM Negative scale, reflecting theory-of-mind-related violations of neurotypical conversational norms, were negatively associated with two forms of non-interactive theory of mind: visual-affective and spontaneous. In contrast, the cToM Positive scale, reflecting explicit mental state language and perspective-taking, was not associated with non-interactive theory of mind. Furthermore, autistic youth were rated higher than typically developing youth on cToM Negative, but the groups were rated similarly on cToM Positive. Together, these findings provide insight into multiple aspects of theory of mind in conversation and reveal a nuanced picture of the relative strengths and difficulties among autistic youth.