<p>Flexible strain sensors are essential components for wearable electronics, implantable biointerfaces, and soft human–machine systems. As application scenarios expand from epidermal monitoring toward long-term in vivo operation, increasingly stringent requirements are imposed on multifunctionality, mechanical compliance, signal stability, and biointegration. Carbon-based functional materials, owing to their tunable electrical properties, structural versatility, and favorable electromechanical compatibility, have emerged as a central materials platform for next-generation flexible strain sensors. This review presents a comprehensive, mechanism-oriented overview of carbon-enabled flexible strain sensing, encompassing piezoresistive, capacitive, and piezoelectric transduction modes. This review systematically examine how carbon material dimensionality including low-dimensional nanofillers, two-dimensional sheets, and three-dimensional porous, governing sensitivity, durability, and long-term device reliability. Particular emphasis is placed on contrasting the fundamentally different design requirements of wearable and implantable systems, including sensitivity-stability trade-offs, tissue-level mechanical matching, and operational robustness in complex biological environments. Distinct from prior material- or device-centric reviews, this review establishes a unified framework linking carbon architectures, sensing mechanisms, and application contexts, thereby clarifying critical bottlenecks and design principles for advancing multifunctional, biointegrated strain sensors toward practical and translational use.</p>