Functionally graded nanocomposites (FGNCs) are fabricated using 3D printing for hardness, impact and compression investigations with different wt. % (0.5⟶H0.5, 1⟶H1, 3⟶H3, 5⟶H5) of functionalized MWCNTs in high-density polyethylene (HDPE). Further, they are extruded as feedstock filament for 3D printing. The hardness, impact strength, and compressive properties increase while the energy absorption decreases with a % increase in the functionalized MWCNTs. The highest hardness, impact strength, compressive moduli, and strength observed for FGNC-2 (H1⟶H3⟶H5) are 76.80, 119.99, 61.14, and 11.56%, respectively, higher than HDPE. The betterment in the mechanical properties is attributed to the strengthening and stiffening effects due to the homogeneous distribution of the functionalized MWCNTs in HDPE. The FGNC-2 exhibited the highest mechanical properties, and can be used in various applications wherein variational stiffness is required.