Background: Structural mentoring barriers are policies, practices, and cultural norms that collectively disadvantage marginalized groups and perpetuate disparities in mentoring. While these mentoring barriers can be found early in the training pathway, failure to address or overcome these barriers at the postdoctoral training stage has a direct impact on faculty diversity and national efforts to retain underrepresented groups in research careers. Methods: To better understand the mentoring barriers faced by postdoctoral trainees, and possible ways to address them, a diverse sample of postdoctoral scholars ("postdocs") from across the United States were asked to participate in focus groups to discuss their training experiences. We conducted five 90-minute focus groups with 32 biomedical postdocs, including 20 (63%) women and 15 (47%) individuals from underrepresented racial/ethnic groups (URG). Participants were well-represented across years of training, and 65% were at least somewhat likely to pursue a research-intensive faculty career, similar to previously reported national averages. Results: A social ecological framework was used to examine both the upstream and downstream manifestations of structural mentoring barriers, as well as mentor barriers, overall. Themes were categorized on four broad levels: Individual (attitudes, beliefs, knowledge, or behaviors that inform mentoring barriers), Interpersonal (mentoring barriers arising from dyadic, peer, or network relationships), Institutional (departmental, institutional, organizational mentoring barriers), or Systemic (mentoring barriers originating from policies or broad social and cultural norms). Notable structural barriers included (1) academic politics and scientific hierarchy, (2) inequalities resulting from mentor prestige, (3) the (over) reliance on one mentor, (4) the lack of formal training for academic and non-academic careers, and (5) the lack of institutional diversity and institutional mentor training. These structural barriers foster mentoring practices and behaviors that lead to poor work-life balance, poor communication, and research career attrition. To overcome these barriers, postdocs strongly encouraged developing a network or team of mentors and recommended institutional interventions that create more comprehensive professional development, mentorship, and belonging. Conclusions: For postdoctoral scientists, structural mentoring barriers can permeate down to institutional, interpersonal, and individual levels, impeding a successful transition to an independent research career. It has become clear that large-scale changes in mentoring must come from addressing the policies, practices, and cultural norms that perpetuate poor mentoring. This work provides strong evidence for promoting mentorship networks and cultivating a "mentoring milieu" that fosters a supportive community and a strong culture of mentorship at all levels.