Autistic university students face stigma. Online trainings have been used to improve explicit autism stigma (social distance) and knowledge among university students in different countries. However, autistic university students have not typically been involved in developing such trainings. We developed two autism trainings: a participatory training (developed in collaboration with autistic university students) and a non-participatory training. We evaluated these trainings with undergraduate students in the United States and Lebanon. A pilot study revealed improvements in implicit biases (measured with an Implicit Association Test) and knowledge following both trainings, but no clear benefit of the participatory training in particular. Feedback revealed that participants found the Implicit Association Test tedious, suggesting that it might have dampened effects by boring participants. To increase engagement, we removed the Implicit Association Test and conducted a cross-university training comparison which revealed evidence that the participatory training was more effective than the non-participatory training at improving autism knowledge, explicit stigma, and attitudes toward inclusion. Autistic co-authors coded participant feedback and identified three key themes to guide future training development and adaptation: an (inter)personal element, accessibility, and clarity of information. These studies provide empirical support for the oft-cited, but rarely directed tested, benefits of involving autistic people in research about autism.