The objective of the study is to report the feasibility, acceptability, and adoption of the "Access to Tailored Autism Integrated Care (ATTAIN)," a model to identify mental health needs and link to mental health care for autistic children. The Exploration, Preparation, Implementation, and Sustainment framework informed implementation outcome measurement and analysis. Thirty-six providers from seven pediatric primary care clinics within three health care systems enrolled and received an initial Access to Tailored Autism Integrated Care training and ongoing online support over 4 months with autistic patients ages 4--16 years old. Survey and interview assessments measured perceptions of feasibility, acceptability, and intentions to sustain Access to Tailored Autism Integrated Care use. Electronic health record (EHR) data assessed child characteristics and Access to Tailored Autism Integrated Care adoption. Descriptive statistics and one-way analyses of variance characterized implementation outcomes and differences between health care systems. Rapid qualitative methods were used to analyze interview data that were integrated with survey data for convergence and expansion. Providers reported that Access to Tailored Autism Integrated Care was feasible, acceptable, and that the initial training was helpful. Reports were mixed about providers' intentions to continue using Access to Tailored Autism Integrated Care. Providers offered recommendations for more specific and tailored implementation support. There were significant differences in provider-reported adoption rates between health care systems. This is one of the first implementation studies examining integrated mental health care for autistic children and highlighted specific areas for refinement to facilitate scale-up.