Phosphate glass fibers were successfully developed using a mixture of natural phosphate and treated oil shale (TOS), varying from 0 to 20% TOS. The effect of TOS addition on their structural, mechanical, and chemical properties was investigated. These fibers exhibited amorphous behavior, with an increase in density and a decrease in molar volume as TOS content increased. This suggests depolymerization of the glassy network, leading to the breaking of POP bonds and crosslinking of the network with covalent POM bonds. Changes in the infrared and Raman spectra support this observation. At 20% TOS concentration, tensile strength and tensile modulus values reach their maximum at 1716 ± 101 MPa and 91.40 ± 9.13 GPa, respectively. These fibers exhibit the lowest percentage weight loss when exposed to distilled water for both immersion media at 25 °C and 37 °C, reaching 8.74% and 13.73%, respectively, after 120 h. The enhancement of the mechanical and chemical properties of the studied fibers is mainly attributable to the replacement of P–O–P bonds by highly crosslinked P–O–Si, P–O–Al, and P–O–Fe bonds. These results suggest that these fibers could be used as environmentally friendly fertilizers with controlled release of nutrient ions.