Nature has evolved a treasury of biological molecules that are logically connected to networks, enabling cells to maintain their functional integrity. Similar to electronic circuits, cells operate as information-processing systems that dynamically integrate and respond to distinct input signals. Synthetic biology aims to standardize and expand the natural toolbox of biological building blocks to engineer novel synthetic networks in living systems. Mammalian cells harboring integrated designer circuits could work as living biocomputers that execute predictable metabolic and therapeutic functions. This review presents design principles of mammalian gene circuits, highlights recent developments, and discusses future challenges and prospects.