A defining feature of adaptive immunity is the development of long-lived memory T cells to curtail infection. Recent studies have identified a unique stem-like T-cell subset amongst exhausted CD8-positive T cells in chronic infection(1-3)', but it remains unclear whether CD4-positive T-cell subsets with similar features exist in chronic inflammatory conditions. Amongst helper T cells, T(H)17 cells have prominent roles in autoimmunity and tissue inflammation and are characterized by inherent plasticity'(4-7), although how such plasticity is regulated is poorly understood. Here we demonstrate that T(H)17 cells in a mouse model of autoimmune disease are functionally and metabolically heterogeneous; they contain a subset with stemness-associated features but lower anabolic metabolism, and a reciprocal subset with higher metabolic activity that supports transdifferentiation into T(H)1-like cells. These two T(H)17-cell subsets are defined by selective expression of the transcription factors TCF-1 and T-bet, and by discrete levels of CD27 expression. We also identify signalling via the kinase complex mTORC1 as a central regulator of T(H)17-cell fate decisions by coordinating metabolic and transcriptional programmes. T(H)17 cells with disrupted mTORC1 signalling or anabolic metabolism fail to induce autoimmune neuroinflammation or to develop into T(H)1-like cells, but instead upregulate TCF-1 expression and acquire stemness-associated features. Single-cell RNA sequencing and experimental validation reveal heterogeneity in fate-mapped T(H)17 cells, and a developmental arrest in the T(H)1 transdifferentiation trajectory upon loss of mTORC1 activity or metabolic perturbation. Our results establish that the dichotomy of stemness and effector function underlies the heterogeneous T(H)17 responses and autoimmune pathogenesis, and point to previously unappreciated metabolic control of plasticity in helper T cells.