Polylactic acid (PLA) has increasingly attracted research in various industrial fields due to its great biocompatibility and sustainability over other thermoplastics, which are widely used as filament feedstock in 3D-printing technology, specifically in Fused Deposition Modelling (FDM). Despite PLA being suitable in FDM processing, it has limitations in applications that need plastic deformation at high-stress levels due to its low strength and ductility. For this purpose, this review article discusses the existing studies that involve the incorporation of fillers in 3D-printed polylactic acid to maximize its functionality, which is non-attainable by the pure filament material alone. An overview of polylactic acid in FDM and the properties and effects of functional fillers of different types are presented. Finally, a complete table of which functional fillers are categorized (carbonaceous, metallic, ceramic and glassy, plant-based, and mineral) summarizes the reported comparison of 3D-printed pure PLA and the composite, scoping to reveal the mechanical modifications of each filler.