Salt stress is a key abiotic factor that inhibits mustard plant growth and grain yield, owing to oxidative damage, osmotic imbalance, and disturbances in metabolic activities. Salicylic acid (SA) and potassium (K) have been identified as crucial components in regulating salt stress and improving plant tolerance. SA and potassium K function as reactive oxygen species (ROS) scavengers, contributing to cellular water balance by reducing oxidative damage and facilitating osmotic adjustment. Additionally, they improve nitrogen assimilation, which leads to an increase in proline and protein content. This reinforces the plant’s ability to withstand salt stress. The present study revealed the significant potential of SA and K, individually or in combination, to alleviate the adverse effects of salt stress on mustard plants. This treatment significantly improved nitrate reductase activity by 27.72%. Additionally, the combination enhanced the activity of the antioxidant enzymes activities, including SOD, CAT, and APX contents increased by 93.94%, 37.77%, and 151.51% respectively. The accumulation of total phenolic compounds (TPC) and total flavonoid contents (TFC) was significantly increased, both essential for preventing oxidative damage. These findings indicate the synergistic effect of SA and K in strengthening different physiological and biochemical processes required for salt tolerance. The increase in these pathways shows that SA and K mitigate adverse stress effects and prepare plants for enhanced metabolic efficiency under stressful conditions. For instance, the stability of cells is enhanced by an increase in proline and protein content, while the increased activity of antioxidant enzymes ameliorates the damage to cells caused by reactive oxygen species (ROS). The significant involvement of secondary metabolites in stress adaptation is further emphasized by the increased accumulation of TPC and TFC.