Background/Objectives: The utilization of amphiphilic Pt(IV) complexes as prodrugs offers a promising strategy to revolutionize Pt-based cancer therapy by enhancing drug delivery and activation. While PEGylation is widely used to optimize drug properties, its impact on the biological behavior of amphiphilic Pt(IV) complexes remains unclear. This study systematically investigates how the PEGylation of varying molecular weights influences their cytotoxicity, cellular uptake, and activation. Methods: Pt(IV) complexes were synthesized with PEG chains of different molecular weights using HATU-catalyzed amide bond formation and copper-free click chemistry. Their biological properties were assessed through cell-based analyses, focusing on cytotoxicity, cellular uptake, and activation by biological reductants. Results: Small PEG modifications retained the potent cytotoxicity of amphiphilic Pt(IV) prodrugs, whereas large PEG chains significantly reduced efficacy. The decrease in potency was linked to impaired cellular uptake and mitochondrial accumulation. Additionally, large PEG modifications slowed the reduction and activation of Pt(IV) prodrugs by biological reductants, further limiting their anticancer activities. Conclusions: These findings underscore the critical role of PEGylation in metallodrug design and provide key insights into optimizing PEGylation strategies for enhancing platinum-based cancer therapies.