Beilenson was elected to the California State Assembly in 1963, serving until 1967, and then served in the California State Senate from 1967 to 1976. Among his accomplishments in the California State Legislature was winning enactment of the "Beilenson Act", which requires public hearings whenever hospitals in California are closed or reduce services. His most noteworthy accomplishment was as author of the 1967 Therapeutic Abortion Act, one of the most liberal abortion laws at that time, which legalized abortion when a woman’s mental or physical health was at risk or if pregnancy resulted from rape or incest.