The degree of lineage stability achieved by pathogen-specific CD4⁺ T cells in vivo, and how this impacts host defense against infection, remains unclear. We demonstrate that in response to Th1-polarizing intracellular bacterial or viral pathogens, only 80%–90% of responding polyclonal T cells become indelibly committed to this lineage. Th1 commitment was nearly invariant in cells that proliferated extensively, but perturbations to the extrinsic cytokine milieu or the pathogen's ability to enter the cytosol impeded commitment and promoted plasticity for future IL-17 expression. Conversely, cell-intrinsic interferon-γ expression and acquisition of permissive chromatin at the Ifng gene during priming predicted heritable Th1 commitment. Importantly, CD4⁺ T cells that retained plasticity conferred protection against Mycobacterium tuberculosis, while these protective effects were abolished with Th17 polarization. These findings illustrate the immune signals that induce memory CD4⁺ T cell responses required for maintaining host defense against infection yet are adaptable in novel environmental contexts.