To increase the printing and mechanical performance attributes of three-dimensional-printed concrete (3DPC), fiber addition has proven to be a highly effective method. Among various fiber types, glass fibers have been explored for use in 3DPC because of their favorable properties and affordability. However, detailed studies on the impacts of glass fibers, especially the impact of fiber length on the performance of 3DPC, remain limited. In this research, 3D-printed mortar (3DPM) mixtures with varying water-to-cement (W/C) ratios (0.24–0.32) and glass fiber lengths (5.00–25.00 mm) were prepared to examine their printability and mechanical performance. The findings revealed that a suitable reduction in the W/C ratio positively influences printability and strength. Crucially, an increase in the glass fiber length notably increased the extrudability, dimensional stability, and buildability, increasing the flexural strength by approximately 75.0% but causing a maximum decrease in the compressive strength of approximately 22.0%. A comparison of the strengths of the printed and casted samples revealed that the extrusion and stacking processes had a profound influence on both the flexural and compressive strengths, with the flexural strength potentially increasing by 92.6% and the compressive strength decreasing by up to 46.8%. This suggests that engineering applications of glass fiber-reinforced 3DPC shall consider decreased compressive strength, and methods such as reducing the W/C ratio and using strength-boosting admixtures may be applied.