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Targeting cells for ablation, gene therapy or drug delivery is an ongoing challenge in development of therapeutics. With the goal of efficiently labeling cell surfaces based on target recognition, we linked the well-established engineered ultraID biotin ligase to cell-surface affinity reagents to enable catalytic installation of tags on the surface of cells. Biotin ligase conjugated to nanobodies against cell surface proteins exhibited an order of magnitude increase in cell labeling compared to nanobody alone. Biotin labeling of cells remained efficient at low nanomolar concentrations of the conjugate, and was stable for at least 4 h. By testing a swath of antibody-biotin ligase conjugates against various targets and cell types, we confirmed the feasibility and generalizability of this approach. Administration of biotin ligase-conjugated antibodies into animals efficiently biotinylated the expected target cells. Biotinylated target cells were subsequently detectable with fluorescent streptavidin injected into the same mice. Enzyme mediated cell targeting thus provides an efficient and modular platform for biotinylation and detection of cells expressing specific surface targets with potential applications for research and therapeutics.