An assurance case is a structured hierarchy of claims aiming at demonstrating that a mission-critical system supports specific requirements (e.g., safety, security, privacy). The presence of assurance weakeners (i.e., assurance deficits, logical fallacies) in assurance cases reflects insufficient evidence, knowledge, or gaps in reasoning. These weakeners can undermine confidence in assurance arguments, potentially hindering the verification of mission-critical system capabilities which could result in catastrophic outcomes (e.g., loss of lives). We report the first comprehensive systematic mapping study on assurance weakeners. We followed the well-established PRISMA 2020 and SEGRESS guidelines to conduct our systematic mapping study, searching for primary studies in five digital libraries and focusing on the 2012–2023 publication year range. Our selection criteria focused on studies addressing assurance weakeners from a qualitative standpoint, resulting in the inclusion of 39 primary studies. Our systematic mapping study reports a taxonomy (map) that provides a uniform categorization of assurance weakeners and approaches proposed to manage them from a qualitative perspective. The taxonomy classifies weakeners in four categories: aleatory, epistemic, ontological, and argument uncertainty. Additionally, it classifies approaches supporting the management of weakeners in three main categories: representation, identification, and mitigation approaches. Our study findings suggest that the SACM (Structured Assurance Case Metamodel), —a standard specified by the OMG (Object Management Group)—, offers a comprehensive range of capabilities to capture structured arguments and reason about their potential assurance weakeners. Our findings also suggest novel assurance weakener management approaches should be proposed to better assure mission-critical systems.