Supporting pupils with autism spectrum disorder (ASD) in mainstream schools is a challenging task. This article proposes a professional development framework for educational psychologists (EPs) to consider when supporting the development of specialist ASD staff. The framework focuses on training content, educator characteristics and organisational elements. Nine mainstream schools developing additional provision to support children with ASD and specific language impairment (SLI) participated in the research. Specialist staff were provided with training and took part in follow up interviews about their developing practice. A pre- and post-questionnaire of participants (N = 30) attending the specialist training is supplemented with longitudinal interview data from specialist staff (N = 20). A paired sample t-test of questionnaire data showed that staff self-efficacy significantly increased pre- to post-training with a large effect size (0.61). Thematic analysis of interviews provided evidence of sustained professional development over time, underpinned by supportive organisational factors.