Light-matter interactions lie at the heart of condensed matter physics, providing physical insight into material behaviour while enabling the design of new devices. Perhaps this is most evident in the push to develop quantum information and spintronic technologies. On the side of quantum information, engineered light-matter interactions offer a means to drastically enhance coherence rates, while at the same time new insights into spin-photon manipulation would benefit the development of spintronic technologies. In this context the recent discovery of hybridization between ferromagnets and cavity photons has ushered in a new era of light-matter exploration at the crossroads of quantum information and spintronics. The key player in this rapidly developing field of cavity spintronics is a new type of quasiparticle, the cavity-magnon-polariton. In this dissertation we explore the fundamental behaviour of the cavity-magnon-polariton and exploit its unique properties to develop new spintronic applications. To understand the physical origins of spin-photon hybridization we develop a comprehensive theoretical framework, relating the basic characteristics of hybridization to a universal model of coupled oscillators, revealing the physical origin of the coupling through electrodynamic phase correlation, and describing detailed properties through a quantum approach. Based on this foundation we have performed in depth experimental investigations of the coupled spin-photon system. We discover that the coupling will influence spin current generated through the spin pumping mechanism, demonstrating a firm link between spin-photon coupling and spintronics. We also develop several in-situ coupling control mechanisms, which offer both physical insight and a means to develop cavity spintronic technologies. As one example, we have combined our local spin current detection technique and coupling control mechanism to realize non-local spin current manipulation over distances of several centimetres. Therefore by revealing the electrodynamic nature of strong spin-photon coupling, developing new control mechanisms, and demonstrating the influence on spin current, this dissertation sets the foundation of cavity spintronics, opening the door to the implementation of strong spin-photon coupling for new spintronic and quantum information technologies.