The lightweighting of 3D-printed components is achievable by using infill patterns and the ability to adjust their density. In this context, performing a mechanical characterization and numerical simulation of the printed parts is imperative. This manuscript conducts experimental and numerical investigations on 3D-printed composites (onyx/glass fibers) that consider the infill pattern, walls, roofs, and floors of the samples. A numerical homogenization approach was adopted to identify the elastic mechanical parameters of the infill patterns. The results demonstrated the homogenization tool’s effectiveness in predicting the mechanical parameters of the infill patterns. Relationships correlating the infill density and each homogenized mechanical parameter were established, enabling the calculation of each mechanical parameter based on the used infill pattern and its density without reiterating the mechanical homogenization. Regarding the simulation of specimens under tension and flexure, the results indicated that the prediction error of the elastic modulus ranged between 2.87% and 11.84% for tension and between 4.42% and 8.45% for 3-point bending. The simulation of 3D-printed composites, considering all constituent elements of the specimens, allowed for examining stress fields in each element and identifying areas of highest and lowest stress. These findings can contribute to predicting the behavior of 3D-printed composites in the context of addressing engineering problems.