Tumor-initiating cells (TICs) reprogram their metabolic features to meet their bioenergetic, biosynthetic, and redox demands. Our previous study established a role for wild-type isocitrate dehydrogenase 1 (IDH1 ^WT ) as a potential diagnostic and prognostic biomarker for non–small cell lung cancer (NSCLC), but how IDH1 ^WT modulates NSCLC progression remains elusive. Here, we report that IDH1 ^WT activates serine biosynthesis by enhancing the expression of phosphoglycerate dehydrogenase (PHGDH) and phosphoserine aminotransferase 1 (PSAT1), the first and second enzymes of de novo serine synthetic pathway. Augmented serine synthesis leads to GSH/ROS imbalance and supports pyrimidine biosynthesis, maintaining tumor initiation capacity and enhancing gemcitabine chemoresistance. Mechanistically, we identify that IDH1 ^WT interacts with and stabilizes PHGDH and fragile X–related protein-1 (FXR1) by impeding their association with the E3 ubiquitin ligase parkin by coimmunoprecipitation assay and proximity ligation assay. Subsequently, stabilized FXR1 supports PSAT1 mRNA stability and translation, as determined by actinomycin D chase experiment and in vitro translation assay. Disrupting IDH1 ^WT -PHGDH and IDH1 ^WT -FXR1 interactions synergistically reduces NSCLC stemness and sensitizes NSCLC cells to gemcitabine and serine/glycine–depleted diet therapy in lung cancer xenograft models. Collectively, our findings offer insights into the role of IDH1 ^WT in serine metabolism, highlighting IDH1 ^WT as a potential therapeutic target for eradicating TICs and overcoming gemcitabine chemoresistance in NSCLC.