Protein phosphorylation signaling networks have a central role in how cells sense and respond to their environment. This thesis outlines a comprehensive approach to designing and implementing synthetic phosphorylation networks in mammalian cells, using modular protein domain parts to construct reversible phosphorylation cycles and assemble customizable circuits. By leveraging model-guided tuning, these engineered circuits enable precise signal processing and facilitate the creation of diverse network connections. The synthetic pathways can be linked to upstream cell surface receptors for rapid sensing of extracellular ligands and downstream elements that regulate gene expression. The work further explores the application of these synthetic networks in therapeutically relevant settings. We demonstrate how engineered circuits can detect physiologically significant biomolecules, such as inflammation markers, and respond with targeted actions, including the controlled secretion of therapeutic proteins. The successful integration and functional testing of these synthetic pathways in primary human cells highlight a significant step toward their use in cell-based therapies. This adaptability illustrates the potential for engineering customized cellular responses tailored to specific disease states, paving the way for innovative treatment strategies. By providing a robust toolkit and showcasing its versatility, this thesis lays the groundwork for future advancements in synthetic biology. The modular design and adaptability of these synthetic signaling networks create opportunities for developing programmable cellular systems capable of addressing a wide range of biotechnological and medical challenges. This work contributes to the growing field of synthetic biology by establishing a foundational framework for integrating engineered pathways into cellular systems, enhancing their ability to perform complex, tailored functions and expanding the scope of potential applications in biosensing and therapeutic development.