PURPOSE: Demand for germline hereditary cancer genetic testing has increased because of reduced cost, gene discovery, expanding indications, and precision cancer therapies. The traditional model for germline testing, where a genetics provider performs all steps of the testing process (pretest counseling, test ordering, results disclosure, and post-test counseling), is no longer able to meet testing needs especially for patients with cancer needing timely germline testing for treatment decisions. Mainstreaming has emerged as an alternative approach to increase testing capacity and efficiency, where nongenetics providers perform these steps and genetics providers focus on post-test counseling for positive results. METHODS: This study reports a 3-year experience with mainstreaming hereditary cancer gene panel testing at Kaiser Permanente Southern California. The study compared demographic characteristics, cancer diagnoses, and test results between patients tested by genetics providers (traditional model) versus nongenetics providers (mainstreaming) over 3 years. Over 32,000 germline hereditary cancer gene panels were completed, including nearly 12,000 mainstreaming tests. RESULTS: Mainstreaming substantially increased testing volume. Patients undergoing mainstream testing were more likely to have cancer, be male, and self-report being Asian or Black. The positive test rate was slightly lower in the mainstreaming group (11%) compared with the traditional testing model (15%), with similar rates of variants of uncertain significance. Post-test genetic counseling was high in both groups for positive results. CONCLUSION: This study demonstrates that mainstreaming can be successfully implemented in a large health care system and significantly expand testing capacity.