Mixed-species groups are common amongst diverse taxa including fishes, birds, and mammals. Antipredator, foraging, and social benefits have been proposed as functional explanations for mixed-species group formation. Amongst delphinids, mixed-species groups are widespread, but little is known about their function. To investigate the potential benefits of delphinid mixed-species groups, we compared the number of individuals, the age composition, and the behaviour of single- and mixed-species sightings of Australian humpback (Sousa sahulensis) and Indo-Pacific bottlenose dolphins (Tursiops aduncus) observed around the North West Cape, Western Australia. We found no difference in the number of individuals or the age composition of humpback dolphins present in single- and mixed-species sightings, whereas bottlenose dolphins were present in larger numbers in single-species sightings than in mixed-species sightings due to a higher number of adults. Socialising was the initial observed behavioural state of 36.1% of mixed-species sightings, compared to only 5.1% and 10.3% of humpback and bottlenose dolphin single-species sightings, respectively. Furthermore, both species travelled and foraged less frequently when in mixed-species groups. Of 93 mixed-species groups observed during a focal follow of ≥10 minutes, 32 (34.4%) involved aggressive and/or sexual behaviours typically initiated by bottlenose dolphins towards humpback dolphins while the remaining 61 (65.6%) involved only neutral and affiliative behaviours. The results of this study suggest that the observed mixed-species groups provide multiple social benefits, particularly those pertaining to socio-sexual behaviours and the development and care of young.