We develop a first-principles framework for magnetoelastic coupling in two-dimensional magnets based on a strain-dependent Heisenberg model. In this approach, strain derivatives of the exchange interactions provide direct access to magnetostriction and to the magnetic renormalization of the elastic tensor, establishing a microscopic link between spin interactions and elastic response. We apply the method to monolayer NiPS₃ and CrPS₄, which exhibit contrasting magnetoelastic behavior. NiPS₃ shows weak and nearly isotropic spin-lattice coupling, consistent with a robust zigzag antiferromagnetic ground state. In contrast, CrPS₄ displays strong anisotropic coupling, leading to strain-driven transitions between spin-spiral and ferromagnetic phases and significant changes in the critical temperature and elastic response. Our results demonstrate a general route to quantify magnetoelastic effects in low-dimensional magnets and highlight CrPS₄ as a promising platform for strain engineering of magnetic order.