Under certain exposure conditions, a femtosecond laser beam focused in the bulk of fused silica leads to the formation of self-organized structures consisting of a series of “nanolayers,” parallel to one another. Remarkably, this laser-induced nanoscale anisotropy offers the possibility to locally engineer macroscopic properties of a given substrate by selectively exposing it in arbitrarily chosen locations to a laser beam with designed polarization states. Although various physical properties are affected by the laser, this paper specifically discusses in-plane elastic properties of these nanostructures. Using a method based on monitoring resonant properties of vibrating cantilevers combined with a mechanical model of the nanostructures, the Young's moduli of individual nanolayers are calculated and used to define the stiffness matrix of the composite structure. The model shows a good agreement with measured mechanical properties of arbitrarily oriented nanostructures. This work demonstrates the predictability and controllability of laser-induced nanoscale mechanical properties and offers a framework for engineering arbitrary elastic properties through 3D laser writing.