This study demonstrates the dynamic control of valley polarization in monolayer MoS₂ using the helicity of circularly polarized light. Monolayer MoS₂, a two-dimensional non-centrosymmetric crystal with direct band gaps at two valleys, exhibits valley polarization that can be dynamically controlled by optical pumping. The polarization is retained for over 1 ns, showing the potential for valley-based electronic and optoelectronic applications. The key mechanism involves the coupling between valley and spin degrees of freedom in monolayer MoS₂, where the spin-orbit interaction splits the valence bands and leads to valley-spin coupling. Circularly polarized light with opposite helicity excites different valleys, enabling valley polarization. The study also shows that the valley polarization is significantly different in monolayer and bilayer MoS₂ due to their distinct crystal symmetries. In monolayer MoS₂, the valley and spin are coupled, leading to long-lived valley-spin polarization, while in bilayer MoS₂, the inversion symmetry is restored, decoupling valley and spin and resulting in shorter spin lifetimes. The results highlight the importance of crystal symmetry and spin-orbit interactions in valleytronics. The study also provides insights into the spin relaxation mechanisms in monolayer MoS₂, including the Elliot-Yafet and Dyakonov-Perel mechanisms, and shows that the BAP mechanism is a significant contributor to spin relaxation in this system. The findings have implications for the development of valley-based electronic and optoelectronic devices.This study demonstrates the dynamic control of valley polarization in monolayer MoS₂ using the helicity of circularly polarized light. Monolayer MoS₂, a two-dimensional non-centrosymmetric crystal with direct band gaps at two valleys, exhibits valley polarization that can be dynamically controlled by optical pumping. The polarization is retained for over 1 ns, showing the potential for valley-based electronic and optoelectronic applications. The key mechanism involves the coupling between valley and spin degrees of freedom in monolayer MoS₂, where the spin-orbit interaction splits the valence bands and leads to valley-spin coupling. Circularly polarized light with opposite helicity excites different valleys, enabling valley polarization. The study also shows that the valley polarization is significantly different in monolayer and bilayer MoS₂ due to their distinct crystal symmetries. In monolayer MoS₂, the valley and spin are coupled, leading to long-lived valley-spin polarization, while in bilayer MoS₂, the inversion symmetry is restored, decoupling valley and spin and resulting in shorter spin lifetimes. The results highlight the importance of crystal symmetry and spin-orbit interactions in valleytronics. The study also provides insights into the spin relaxation mechanisms in monolayer MoS₂, including the Elliot-Yafet and Dyakonov-Perel mechanisms, and shows that the BAP mechanism is a significant contributor to spin relaxation in this system. The findings have implications for the development of valley-based electronic and optoelectronic devices.