Proteins are often tagged for visualization or delivery in the "sea" of other macromolecules in cells but how tags affect protein mobility remains poorly understood. Here, we employ in-cell F-19 NMR to quantify the mobility of proteins with charged tags in Escherichia coli cells and Xenopus laevis oocytes. We find that the transient charge-charge interactions between the tag and cellular components affect protein mobility. More specifically, positively charged tags impede protein mobility.