Tailoring the properties of high-performance polymers through reinforcing will bring multifunctionality and expand their use in additive manufacturing (AM). However, machine and material-based challenges exist, eventually resulting in low-quality end products. When expensive polymers are considered, it is still challenging to tailor their properties and print them for high-quality multifunctional structures. Here, polymer composites of polyetherimide (PEI) and carbon nanotubes (CNTs) with varying CNT weight fractions are produced in filament form by melt-processing. Neat PEI, 1 wt% and 3 wt% CNTs/PEI are additively manufactured in two different raster orientations (Rectilinear and Concentric), and two most proposed testing geometries, ASTM D638 and D3039. Through effective melt-processing, CNT-reinforced PEI filaments were achieved, printing parameters and testing protocols were discussed. As a result, ASTM D3039 showed superior coherence with filaments’ mechanical properties. Moreover, based on failure modes, ASTM D3039 provided better compatibility to AM, owing to its simple rectangular form yielding well-adhered layers.