A family of biodegradable, biocompatible, water soluble cationic polymer precursor, arginine-based unsaturated poly (ester amide) (Arg-UPEA), is reported. Its incorporation into conventional Pluronic diacrylate (Pluronic-DA) to form hybrid hydrogels for a significant improvement of the biological performance of current synthetic hydrogels is shown. The gel fraction (Gf), equilibrium swelling ratio (Qeq), compressive modulus, and interior morphology of the hybrid hydrogels as well as their interactions with human fibroblasts and bovine endothelial cells are fully investigated. It is found that the incorporation of Arg-UPEA into Pluronic-DA hydrogels significantly changes their Qeq, mechanical strength, and interior morphology. The structure–property relationship of the newly fabricated hybrid hydrogels is studied in terms of the chemical structure of the Arg-UPEA precursor, i.e., the number of methylene groups in the Arg-UPEA repeating unit. The results indicate that increasing methylene groups in the Arg-UPEA repeating unit increases Qeq and decreases the compressive modulus of hydrogels. When compared with a pure Pluronic hydrogel, the cationic Arg-UPEAs/Pluronic hybrid hydrogels greatly improve the attachment and proliferation of human fibroblasts on hydrogel surfaces. A bovine aortic endothelial cells (BAEC) viability test in the interior of the hydrogels shows that the positively charged hybrid hydrogels can significantly improve the viability of the encapsulated endothelial cell over a 2 week study period when compared with a pure Pluronic hydrogel.