In adulthood, starting a new job is a major life event that, for many, accompanies significant changes to one's personal life (e.g., moving to a new location, setting up new childcare or eldercare arrangements, renegotiating schedules and nonwork responsibilities with a spouse or partner). Research shows that job candidates anticipate the degree of work-family support and conflict they might experience in a new role when deciding to accept or reject a job offer. Despite this, work examining associations between newcomer work-to-nonwork conflict (WNC), once arriving at a new job, and their adjustment to the new work role has lagged. To address this, the current study investigates the relationship between newcomers' work nonwork demands (i.e., WNC) and resources (having a family-supportive supervisor) during organizational entry, in relation to work withdrawal. Results from surveys administered to newcomers across three time points, indicate that newcomers' WNC was positively related to work withdrawal via reduced work-to-nonwork self-efficacy. Additionally, the indirect relationship between WNC and newcomer withdrawal was moderated by family-supportive supervisor behavior, indicating that managers can serve as resources with powerful potential to counteract the negative effects of conflict during this early stage. This study is among the first to explicitly link the work-nonwork and organizational socialization literatures. Our results suggest that organizations aiming to support and retain new workers may benefit from training supervisors to help newcomers manage WNC when starting a new job.