Quantum biology is an exciting and broad field that tries to explain biological phenomena that cannot be treated via classical approaches. One interesting area of this blooming field is magnetoreception, the ability of biological systems to sense and use the Earth’s magnetic field for navigation and other functions. It is important to have a deep understanding of the mechanisms behind magnetoreception both for its fundamental interest and because of potential technological applications such as bio-inspired magnetosensors. Several models have been developed to explain magnetoreception in biological systems, and the most prominent one among them is the radical-pair mechanism (RPM), which is based on the quantum dynamics of electron and nuclear spins in pairs of radical molecules. The premier candidate for magnetoreception based on the RPM involves the protein cryptochrome. In this process, blue light triggers an electron transfer between flavin adenine dinucleotide (FAD) and a tryptophan triad, leading to the formation of a radical pair. While cryptochrome has historically been the main candidate for RPM magnetoreception, recent experimental studies in vivo and in vitro suggest that FAD alone might also act as a biological magnetosensor. For further evaluation of these results, a detailed theoretical model for the observed magnetic field effects (MFEs) on FAD is needed. Previous models of FAD’s photochemistry under magnetic fields were based on semi-classical approaches. In this thesis, we develop a detailed quantum theoretical model that predicts the MFEs on the photochemistry of FAD for all magnetic field magnitudes. Our quantum theoretical model for the RPM in FAD incorporates Zeeman, hyperfine, and exchange interactions, with the exchange interaction depending on the distance between the radicals. We used existing molecular dynamics (MD) simulations to determine the dynamic distances between the radicals in FAD over time, which we then integrated into a quantum master equation to calculate the spin dynamics under different magnitudes of magnetic fields. This quantum-based model, in contrast to previous semi-classical approaches which were limited to only low and high magnetic fields, can predict the full magnetic field dependence of MFEs. Our theoretical results of the MFEs on FAD’s photochemistry presented in this thesis are in quantitative agreement with experimental results on the transient absorption of this molecule at physiological pH values in the absence and presence of an external magnetic field.