Corners are among the most common yet least understood features in material extrusion additive manufacturing (MEAM). They exist in almost all toolpaths, especially lattices and infills, impacting structural integrity and shear flow, especially for fibre composites. However, corners have been rarely quantified as microstructure-governing mechanical entities. This is the first microstructure-mechanics study for 3D‑printed corners, revealing how corner geometries dynamically dictated crack growth and direction, stress concentration, cyclic degradation, and fibre kinematics.Multi-layer single-filament corners with varied angles were 3D printed and tested. For monotonic tensile, highly similarly force-displacement responses of pure and short-fibre-reinforced polymers confirmed material-independent structure-dominated corner mechanics: force reduction and brittle-to-flexible transition with turn angles. For short fibre composites, corner microstructural evolution was identified with dynamic crack and fibre behaviours. Displacement-controlled cyclic tensile showed displacement dependency on self-stiffening and degradation of force, energy dissipation and increase of stiffness. The displacement-control cyclic tensile also revealed several competing mechanisms for fibre reinforced composites (fibre re-orientation and strain-hardening versus damage accumulation). Force-controlled cyclic tensile showed decreased with increasing turn angles, and transition from rupture of small turn angles to delayed rupture for greater angles with hinge-opening. Next, compared to angular corners, rounded corners showed that curvature effectively suppressed stress concentration and improved strength (>7-fold for normalised force), supported by a case study of zigzag lattices with rounded corners (>48% and >87% increase in strength and stiffness).This study highlights corners in MEAM from incidental geometry into primary microstructural features that govern performance, revealing how corner‑induced microstructures could govern load transfer, deformation modes, failure evolution, and fatigue. These provide an insight of corner‑aware toolpath strategies that enhance structural integrity and mitigate mechanical weakness or stress concentration in MEAM.