<p>Artificial enzymes constructed by introducing abiological catalytic cofactors into protein scaffolds are emerging as promising biocatalysts for reactions that have no natural equivalents. However, the design of artificial enzymes for <i>in cellulo</i> stereoselective biosynthesis remains largely unexplored because of the numerous challenges posed by the complex cellular environment. Here, we present an efficient strategy for the intracellular assembly of artificial enzymes via covalent anchoring by incubating cells that are expressing target protein with a synthetic catalytic cofactor, in the manner of natural enzymes. The artificial enzyme is assembled efficiently via site-specific disulfide-bond formation in cells, and it catalyzes an asymmetric Mannich reaction <i>in cellulo</i> with excellent enantioselectivity and reactivity. Crystallographic analysis combined with computational studies provides structural and mechanistic insight into the stereoselectivity and catalytic proficiency. Overall, this work establishes a versatile and generalizable approach to construct artificial enzymes <i>in cellulo</i>, expanding the toolkit for tailored synthetic applications in cellular contexts.</p>