The pervasive presence of pharmaceuticals and personal care products (PPCPs) in aquatic systems necessitates advanced oxidation processes for their effective removal. While perovskite oxides are promising peroxymonosulfate (PMS) activators, research has predominantly focused on B-site optimization, leaving the strategic potential of A-site nonstoichiometry largely unexplored. This study demonstrates that precise A-site engineering in LaxCoO3 perovskites is a highly effective strategy for tailoring catalytic performance. We reveal that a slight La excess (x = 1.05) induces surface La enrichment and optimizes the Co electronic structure, leading to a reduced Co valence state and elongated CoO bonds. This unique configuration fosters a synergistic radical and non-radical (electron transfer) pathway for PMS activation. In contrast, La-deficient (x < 1) and excessively La-rich (x = 1.1) compositions suffer from surface depletion and detrimental La2O3 phase segregation, respectively. The optimized La1.05CoO3 catalyst achieves superior degradation and mineralization of paracetamol and various antibiotics, exhibiting enhanced kinetics, exceptional stability in complex water matrices, and high PMS utilization efficiency. Combined experimental and theoretical analyses confirm that the surface La species promote PMS adsorption, while the modulated electronic structure facilitates efficient electron transfer. This work establishes A-site nonstoichiometry as a fundamental and powerful design lever for developing high-performance perovskite catalysts in environmental remediation.