Thermoplastic filaments used in fused deposition modelling (FDM) are increasingly adopted for functional components, yet their durability under thermal and moisture-related service conditions remains incompletely characterised. This study presents a multi-modal investigation of the mechanical degradation of three commercial FDM filaments, namely polylactic acid (PLA), polyethene terephthalate glycol (PETG), and wood-filled PLA (PLA-Wood), subjected to two independent degradation pathways: accelerated isothermal thermal ageing at material-relevant glass-transition temperatures for four and eight days, and immersion in distilled water for 24–720 h. Specimens were fabricated according to American Society for Testing and Materials (ASTM) D638, ASTM D790, and ASTM D695 standards using 70% concentric infill, then evaluated through tensile, flexural, and compressive testing, gravimetric mass and moisture analysis, and scanning electron microscopy (SEM). Under thermal ageing, PETG exhibited the highest tensile strength retention, maintaining 40.87 ± 1.18 MPa after eight days, with the smallest proportional tensile modulus reduction of ≤ 5.1%. PLA-Wood sustained the greatest compressive modulus loss of 15.6%, attributed to combined PLA matrix chain scission and hygroscopic wood-filler desorption. Under moisture exposure, PETG maintained stable wet-state tensile strengths of 24.38–25.60 MPa after initial conditioning, whereas PLA-Wood absorbed approximately 50% apparent moisture in compression specimens and suffered a 57% flexural strength reduction. PLA showed transient increases in stiffness associated with secondary crystallisation before degradation. SEM confirmed distinct material–stressor degradation pathways. These results establish the first integrated dual-stressor degradation profile for this filament triad, providing quantitative selection criteria for FDM applications involving thermal or humid service environments.