Identification of the factors critical to the tumor-initiating cell (TIC) state may open new avenues in cancer therapy. Here we show that the metabolic enzyme glycine decarboxylase (GLDC) is critical for TICs in non-small cell lung cancer (NSCLC). TICs from primary NSCLC tumors express high levels of the oncogenic stem cell factor LIN28B and GLDC, which are both required for TIC growth and tumorigenesis. Overexpression of GLDC and other glycine/serine enzymes, but not catalytically inactive GLDC, promotes cellular transformation and tumorigenesis. We found that GLDC induces dramatic changes in glycolysis and glycine/serine metabolism, leading to changes in pyrimidine metabolism to regulate cancer cell proliferation. In the clinic, aberrant activation of GLDC correlates with poorer survival in lung cancer patients, and aberrant GLDC expression is observed in multiple cancer types. This link between glycine metabolism and tumorigenesis may provide novel targets for advancing anticancer therapy. Display omitted ► Non-small cell lung cancer (NSCLC) tumor-initiating cells express high levels of glycine decarboxylase (GLDC) ► GLDC is an oncogene that promotes cellular transformation ► GLDC activity regulates pyrimidine metabolism in cancer cells ► GLDC expression predicts mortality in NSCLC patients An enzyme involved in glycine metabolism acts as an oncogene to drive cancer stem cell proliferation and tumorigenesis in non-small cell lung cancer. Aberrant expression of this enzyme occurs in many cancers and correlates with mortality in lung cancer.