Organisms routinely produce minerals, called biominerals, with altered materials properties compared to geologic or traditional lab-grown crystals that are suited to the organism’s needs. By manipulating the mineral growth, the organisms can alter the crystal structure, shape, orientation, and composition, which collectively determine the material’s properties. For example, the mollusk grows its calcium carbonate shell in a gel-like organic template, helping transform the fragile mineral into a stronger material that is hard to break. In another example, a certain class of bacteria uses nanoscale confinement to grow a specific size and shape of iron oxide nanoparticles, which the bacteria can use as a compass to navigate along the Earth’s magnetic field. By translating these biomineral growth strategies to the laboratory, we can achieve a high degree of control over crystalline architectures in a wider range of materials systems than organisms can produce. Because many magnetic and electronic properties depend strongly on nano- and microstructure, the ability to grow complex crystalline architectures made of magnetic or electronic materials enables new properties. My Ph.D. research was inspired by the ability of biominerals to form two types of structures: hierarchical structures that are ordered across many length scales and single crystal composites. For the first half of my Ph.D., I developed a model that predicts how control of hierarchical structure, in particular the crystalline texture, can help us engineer harder magnetic materials. I validated this magnetic model for mosaic crystals of hematite and developed growth pathways for another hierarchically-structured magnetic material, bismuth ferrite. For the second half of my Ph.D., I formed a theoretical framework for how nano- and micro-particles incorporate into single crystals during solution crystal growth. Finally, I developed a generalizable physical confinement-based approach to semiconductor/plasmonic nanoparticle composites for photocatalytic applications.