Cell-surface engineering strategies that permit long-lived display of well-defined, functionally active molecules are highly attractive for eliciting desired cellular responses and for understanding biological processes. Current methodologies for the exogenous introduction of synthetic biomolecules often result in short-lived presentations, or require genetic manipulation to facilitate membrane attachment. Herein, we report a cell-surface engineering strategy that is based on the use of a CMP-Neu5Ac derivative that is modified at C-5 by a bifunctional entity composed of a complex synthetic heparan sulfate (HS) oligosaccharide and biotin. It is shown that recombinant ST6GAL1 can readily transfer the modified sialic acid to N-glycans of glycoprotein acceptors of living cells resulting in long-lived display. The HS oligosaccharide is functionally active, can restore protein binding, and allows activation of cell signaling events of HS-deficient cells. The cell-surface engineering methodology can easily be adapted to any cell type and is highly amenable to a wide range of complex biomolecules.