Protein arginine methylation, catalyzed by protein arginine methyltransferases (PRMTs), plays crucial roles in a variety of cellular processes. Mammalian PRMT1 exists in a large protein complex in cells, which has been implied in modulating the regulatory and catalytic properties of this enzyme. Establishment of a mammalian comparative approach will help to identify putative substrates of PRMT1 in an authentic condition. Here, we showed that ectopically expressed PRMT1 in mammalian HEK293 cells not only exhibited catalytic properties comparable to the endogenous enzyme but also existed in a functional complex together with endogenous PRMT1 and thus functioned as an endogenous counterpart. In addition, the measured methylation level of cellular proteins using a tritium-labeled methyl donor was accordingly enhanced upon ectopic expression of PRMT1. Subsequent proteomic analysis with such PRMT1-expressing cells allowed us to identify several known and putative methylated proteins. In vitro methylation of selected proteins, eukaryotic translation initiation factor 4A-I and vimentin, by cellular PRMT1 was shown. Together, we have demonstrated the functional equivalence of ectopically expressed PRMT1 in HEK293 cells and its application to systematically identify the substrate proteins in a mammalian cell context.